diff --git a/src/SMAPI.Web/Views/Shared/_Layout.cshtml b/src/SMAPI.Web/Views/Shared/_Layout.cshtml
index d1930880d..b0390a4be 100644
--- a/src/SMAPI.Web/Views/Shared/_Layout.cshtml
+++ b/src/SMAPI.Web/Views/Shared/_Layout.cshtml
@@ -9,6 +9,23 @@
     <meta charset="utf-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <title>@ViewData["Title"] - SMAPI.io</title>
+    <script>
+        (function () {
+            function applyTheme(mode) {
+                var actual = mode === 'system'
+                    ? (matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light')
+                    : mode;
+                document.documentElement.dataset.theme = actual;
+                document.documentElement.dataset.themeMode = mode;
+            }
+            applyTheme(localStorage.getItem('theme') || 'system');
+            window.setTheme = function (mode) {
+                applyTheme(mode);
+                if (mode === 'system') localStorage.removeItem('theme');
+                else localStorage.setItem('theme', mode);
+            };
+        })();
+    </script>
     <link rel="stylesheet" href="@Url.ContentWithCacheBust("~/Content/css/main.css")" />
     @RenderSection("Head", required: false)
 </head>
@@ -28,6 +45,15 @@
         </ul>
 
         @RenderSection("SidebarExtra", required: false)
+
+        <h4>Theme</h4>
+        <ul id="theme-picker">
+            <li>
+                <a href="#" data-theme-mode="system" onclick="setTheme('system'); return false;">System</a> |
+                <a href="#" data-theme-mode="light" onclick="setTheme('light'); return false;">Light</a> |
+                <a href="#" data-theme-mode="dark" onclick="setTheme('dark'); return false;">Dark</a>
+            </li>
+        </ul>
     </div>
     <div id="content-column">
         <div id="content">
diff --git a/src/SMAPI.Web/wwwroot/Content/css/file-upload.css b/src/SMAPI.Web/wwwroot/Content/css/file-upload.css
index f29d46aa9..2d46aaff5 100644
--- a/src/SMAPI.Web/wwwroot/Content/css/file-upload.css
+++ b/src/SMAPI.Web/wwwroot/Content/css/file-upload.css
@@ -9,7 +9,9 @@
     margin: auto;
     box-sizing: border-box;
     border-radius: 5px;
-    border: 1px solid #000088;
+    border: 1px solid var(--surface-border);
+    background: var(--surface);
+    color: var(--fg);
     outline: none;
     box-shadow: inset 0 0 1px 1px rgba(0, 0, 192, .2);
 }
@@ -21,5 +23,6 @@
     box-shadow: inset 0 0 1px 1px rgba(0, 0, 0, .2);
     cursor: pointer;
     border: 1px solid #008800;
-    background-color: #cfc;
+    background-color: var(--ok-bg);
+    color: var(--fg);
 }
diff --git a/src/SMAPI.Web/wwwroot/Content/css/index.css b/src/SMAPI.Web/wwwroot/Content/css/index.css
index cac83b0fb..af06a25ee 100644
--- a/src/SMAPI.Web/wwwroot/Content/css/index.css
+++ b/src/SMAPI.Web/wwwroot/Content/css/index.css
@@ -4,7 +4,7 @@
 h1 {
     text-align: center;
     font-size: 6em;
-    color: #000;
+    color: var(--fg);
 }
 
 #blurb {
@@ -110,7 +110,7 @@ h1 {
 ** Subsections
 *********/
 .area {
-    background: rgba(0, 170, 0, 0.2);
+    background: var(--area-bg);
     padding: 0 1em 1em 1em;
     margin-bottom: 1em;
 }
diff --git a/src/SMAPI.Web/wwwroot/Content/css/json-validator.css b/src/SMAPI.Web/wwwroot/Content/css/json-validator.css
index 04512cf3e..c9f15fe87 100644
--- a/src/SMAPI.Web/wwwroot/Content/css/json-validator.css
+++ b/src/SMAPI.Web/wwwroot/Content/css/json-validator.css
@@ -61,12 +61,12 @@
 
 .banner.success {
     border-color: green;
-    background: #CFC;
+    background: var(--ok-bg);
 }
 
 .banner.error {
     border-color: red;
-    background: #FCC;
+    background: var(--error-bg);
 }
 
 .save-metadata {
@@ -79,7 +79,7 @@
 ** Validation results
 *********/
 .table {
-    border-bottom: 1px dashed #888888;
+    border-bottom: 1px dashed var(--border-strong);
     margin-bottom: 5px;
 }
 
@@ -89,25 +89,25 @@
 }
 
 .table {
-    border: 1px solid #000000;
-    background: #ffffff;
+    border: 1px solid var(--surface-border);
+    background: var(--surface);
     border-radius: 5px;
     border-spacing: 1px;
     overflow: hidden;
     cursor: default;
-    box-shadow: 1px 1px 1px 1px #dddddd;
+    box-shadow: 1px 1px 1px 1px var(--border);
 }
 
 .table tr {
-    background: #eee;
+    background: var(--surface-alt);
 }
 
 .table tr:nth-child(even) {
-    background: #fff;
+    background: var(--surface);
 }
 
 #output div.sunlight-line-highlight-active {
-    background-color: #eeeacc;
+    background-color: var(--warn-bg);
 }
 
 .footer-tip {
diff --git a/src/SMAPI.Web/wwwroot/Content/css/log-parser.css b/src/SMAPI.Web/wwwroot/Content/css/log-parser.css
index 89efdc0b6..5b0af423a 100644
--- a/src/SMAPI.Web/wwwroot/Content/css/log-parser.css
+++ b/src/SMAPI.Web/wwwroot/Content/css/log-parser.css
@@ -39,12 +39,12 @@ table caption {
 
 .banner.success {
     border-color: green;
-    background: #CFC;
+    background: var(--ok-bg);
 }
 
 .banner.error {
     border-color: red;
-    background: #FCC;
+    background: var(--error-bg);
 }
 
 .save-metadata {
@@ -63,8 +63,8 @@ table caption {
 
 #fix-list li {
     padding: 0.5em;
-    background: #FFC;
-    border: 1px solid #880;
+    background: var(--warn-bg);
+    border: 1px solid var(--warn-fg);
     border-radius: 5px;
     list-style-type: none;
 }
@@ -74,13 +74,13 @@ table caption {
 }
 
 #fix-list li.notice {
-    background: #EEFFEE;
-    border-color: #080;
+    background: var(--notice-bg);
+    border-color: var(--ok-fg);
 }
 
 #fix-list li.important {
-    background: #FCC;
-    border-color: #800;
+    background: var(--error-bg);
+    border-color: var(--error-fg);
 }
 
 #fix-list li::before {
@@ -92,7 +92,7 @@ table caption {
 ** Log metadata & filters
 *********/
 .table, #filters {
-    border-bottom: 1px dashed #888888;
+    border-bottom: 1px dashed var(--border-strong);
     margin-bottom: 5px;
 }
 
@@ -102,13 +102,13 @@ table caption {
 }
 
 .table {
-    border: 1px solid #000000;
-    background: #ffffff;
+    border: 1px solid var(--surface-border);
+    background: var(--surface);
     border-radius: 5px;
     border-spacing: 1px;
     overflow: hidden;
     cursor: default;
-    box-shadow: 1px 1px 1px 1px #dddddd;
+    box-shadow: 1px 1px 1px 1px var(--border);
 }
 
 .mod-entry {
@@ -144,7 +144,7 @@ table caption {
 }
 
 .table tr {
-    background: #eee;
+    background: var(--surface-alt);
 }
 
 #mods span.notice {
@@ -157,12 +157,12 @@ table caption {
 
 #mods span.notice.btn {
     cursor: pointer;
-    border: 1px solid #000;
+    border: 1px solid var(--surface-border);
     border-radius: 5px;
     position: relative;
     top: -1px;
     padding: 0 2px;
-    background: #eee;
+    background: var(--surface-alt);
 }
 
 #mods span.notice.txt {
@@ -190,7 +190,7 @@ table caption {
 }
 
 .table tr:nth-child(even) {
-    background: #fff;
+    background: var(--surface);
 }
 
 #filters {
@@ -219,7 +219,7 @@ table caption {
 }
 
 #filters .filter-error {
-    color: #880000;
+    color: var(--error-fg);
 }
 
 #filters .filter-error,
@@ -232,7 +232,7 @@ table caption {
     position: fixed;
     top: 0;
     left: 0em;
-    background: #fff;
+    background: var(--bg);
     margin: 0;
     padding: 0.5em;
     width: calc(100% - 1em);
@@ -257,14 +257,13 @@ table caption {
 #filters span {
     padding: 3px 1em;
     display: inline-block;
-    border: 1px solid #000000;
+    border: 1px solid var(--error-fg);
     border-radius: 3px;
     font-family: monospace;
     cursor: pointer;
     font-weight: bold;
-    color: #000;
-    border-color: #880000;
-    background-color: #fcc;
+    color: var(--fg);
+    background-color: var(--error-bg);
 
     user-select: none;
 }
@@ -279,16 +278,16 @@ table caption {
 }
 
 #filters span:hover {
-    background-color: #fee;
+    background-color: var(--error-bg-hover);
 }
 
 #filters span.active {
-    background: #cfc;
-    border-color: #008800;
+    background: var(--ok-bg);
+    border-color: var(--ok-fg);
 }
 
 #filters span.active:hover {
-    background: #efe;
+    background: var(--ok-bg-hover);
 }
 
 #filters .pager {
@@ -306,8 +305,8 @@ table caption {
 }
 
 #filters .pager span {
-    background-color: #eee;
-    border-color: #888;
+    background-color: var(--pager-bg);
+    border-color: var(--pager-border);
 }
 
 #filters .pager span.active {
@@ -323,7 +322,7 @@ table caption {
 }
 
 #filters .pager span:not(.disabled):hover {
-    background-color: #fff;
+    background-color: var(--pager-hover);
 }
 
 
@@ -336,15 +335,15 @@ table caption {
 }
 
 #log .trace {
-    color: #999;
+    color: var(--log-trace);
 }
 
 #log .debug {
-    color: #595959;
+    color: var(--log-debug);
 }
 
 #log .info {
-    color: #000;
+    color: var(--fg);
 }
 
 #log .alert {
@@ -370,7 +369,7 @@ table caption {
 }
 
 #log .section-toggle-message {
-    color: blue;
+    color: var(--link);
 }
 
 #log .log-message-text {
@@ -378,7 +377,8 @@ table caption {
 }
 
 #log .log-message-text strong {
-    background-color: yellow;
+    background-color: var(--highlight-bg);
+    color: var(--highlight-fg);
     font-weight: normal;
 }
 
@@ -387,14 +387,14 @@ table caption {
 }
 
 #log tr {
-    background: #fff;
+    background: var(--bg);
 }
 
 #log td {
     padding: 0 1px;
     background: inherit;
-    border-bottom: 1px dotted #ccc;
-    border-top: 2px solid #fff;
+    border-bottom: 1px dotted var(--border);
+    border-top: 2px solid var(--bg);
     vertical-align: top;
 }
 
@@ -415,15 +415,15 @@ table caption {
     display: block;
     position: absolute;
     border-radius: 4px;
-    box-shadow: 1px 1px 2px #ccc;
+    box-shadow: 1px 1px 2px var(--log-tooltip-shadow);
     background: inherit;
-    border: 1px solid #ccc;
-    background: #efefef;
+    border: 1px solid var(--log-tooltip-border);
+    background: var(--log-tooltip-bg);
     padding: 1px 1px 0 1px;
     font-size: 10pt;
     top: -2px;
     left: 2px;
-    color: #000;
+    color: var(--fg);
 }
 
 #log td:last-child {
@@ -445,17 +445,24 @@ table caption {
 
 #os-instructions [role="tablist"] {
     border: 0;
+    border-bottom-color: var(--border);
 }
 
 #os-instructions [role="tab"] {
     display: block;
     border: 0;
+    border-top-color: var(--border);
     position: relative;
 }
 
 #os-instructions [role="tab"][aria-selected="true"] {
     font-weight: bold;
     border-radius: 0 10px 10px 0;
+    background-color: var(--surface-alt);
+}
+
+#os-instructions [role="tab"]:hover:not([aria-selected="true"]) {
+    background-color: var(--surface);
 }
 
 [role="tab"][aria-selected="true"]::after {
diff --git a/src/SMAPI.Web/wwwroot/Content/css/main.css b/src/SMAPI.Web/wwwroot/Content/css/main.css
index 6818a1a78..63bd40aa7 100644
--- a/src/SMAPI.Web/wwwroot/Content/css/main.css
+++ b/src/SMAPI.Web/wwwroot/Content/css/main.css
@@ -1,3 +1,82 @@
+/* theme */
+:root {
+    /* base */
+    --bg: #FFF;
+    --fg: #000;
+    --fg-muted: #888;
+    --border: #CCC;
+    --border-strong: #AAA;
+    --link: #006;
+    --link-muted: #77B;
+
+    /* surfaces */
+    --surface: #f8f9fa;
+    --surface-alt: #eaecf0;
+    --surface-border: #a2a9b1;
+    --area-bg: rgba(0, 170, 0, 0.2);
+
+    /* status (rows, banners, chips, fix-list) */
+    --ok-bg: #BFB;
+    --ok-bg-hover: #efe;
+    --ok-fg: #080;
+    --warn-bg: #FFC;
+    --warn-fg: #880;
+    --error-bg: #FBB;
+    --error-bg-hover: #fee;
+    --error-fg: #880000;
+    --notice-bg: #EEFFEE;
+    --neutral-bg: #BBB;
+
+    /* log parser */
+    --log-trace: #999;
+    --log-debug: #595959;
+    --log-tooltip-bg: #efefef;
+    --log-tooltip-border: #ccc;
+    --log-tooltip-shadow: #ccc;
+    --highlight-bg: yellow;
+    --highlight-fg: inherit;
+    --pager-bg: #eee;
+    --pager-border: #888;
+    --pager-hover: #fff;
+}
+
+[data-theme="dark"] {
+    --bg: #1D1D1D;
+    --fg: #fff;
+    --fg-muted: #999;
+    --border: #333;
+    --border-strong: #444;
+    --link: #8ab4f8;
+    --link-muted: #aab;
+
+    --surface: #232323;
+    --surface-alt: #2c2c2c;
+    --surface-border: #444;
+    --area-bg: rgba(38, 44, 38, 0.9);
+
+    --ok-bg: #1f3a1f;
+    --ok-bg-hover: #244a24;
+    --ok-fg: #4a8a4a;
+    --warn-bg: #3a3a1a;
+    --warn-fg: #aa9b00;
+    --error-bg: #3a1f1f;
+    --error-bg-hover: #4a2424;
+    --error-fg: #ff8a8a;
+    --notice-bg: #1f3a1f;
+    --neutral-bg: #2a2a2a;
+
+    --log-trace: #777;
+    --log-debug: #aaa;
+    --log-tooltip-bg: #2c2c2c;
+    --log-tooltip-border: #555;
+    --log-tooltip-shadow: #000;
+    --highlight-bg: #806000;
+    --highlight-fg: #fff;
+    --pager-bg: #2c2c2c;
+    --pager-border: #666;
+    --pager-hover: #3a3a3a;
+}
+
 /* tags */
 html {
     height: 100%;
@@ -6,6 +85,8 @@ html {
 body {
     height: 100%;
     font-family: sans-serif;
+    background: var(--bg);
+    color: var(--fg);
 }
 
 h1, h2, h3 {
@@ -16,24 +97,24 @@ h1, h2, h3 {
 
 h1 {
     font-size: 1.5em;
-    color: #888;
+    color: var(--fg-muted);
     margin-bottom: 0;
 }
 
 h2 {
     font-size: 1.5em;
-    border-bottom: 1px solid #AAA;
+    border-bottom: 1px solid var(--border-strong);
 }
 
 h3 {
     font-size: 1.2em;
-    border-bottom: 1px solid #AAA;
+    border-bottom: 1px solid var(--border-strong);
     width: 55em;
     max-width: 100%;
 }
 
 a {
-    color: #006;
+    color: var(--link);
 }
 
 /* content */
@@ -48,8 +129,8 @@ a {
     width: calc(100% - 2em);
     max-width: 60em;
     padding: 0 1em 1em 1em;
-    border-left: 1px solid #CCC;
-    background: #FFF;
+    border-left: 1px solid var(--border);
+    background: var(--bg);
     font-size: 0.9em;
 }
 
@@ -58,7 +139,7 @@ a {
 }
 
 .section {
-    border: 1px solid #CCC;
+    border: 1px solid var(--border);
     padding: 0.5em;
     margin-bottom: 1em;
 }
@@ -122,19 +203,19 @@ a {
     min-height: 75%;
     width: 12em;
     background: url("../images/sidebar-bg.svg") no-repeat top right;
-    color: #666;
+    color: var(--fg-muted);
 }
 
 #sidebar h4 {
     margin: 1.5em 0 0.2em 0;
     width: 10em;
-    border-bottom: 1px solid #CCC;
+    border-bottom: 1px solid var(--border);
     font-size: 0.8em;
     font-weight: normal;
 }
 
 #sidebar a {
-    color: #77B;
+    color: var(--link-muted);
     border: 0;
 }
 
@@ -143,13 +224,21 @@ a {
     padding: 0;
     list-style: none none;
     font-size: 0.9em;
-    color: #888;
+    color: var(--fg-muted);
 }
 
 #sidebar li {
     margin-left: 1em;
 }
 
+/* theme picker */
+html[data-theme-mode="system"] #theme-picker [data-theme-mode="system"],
+html[data-theme-mode="light"] #theme-picker [data-theme-mode="light"],
+html[data-theme-mode="dark"] #theme-picker [data-theme-mode="dark"] {
+    font-weight: bold;
+    text-decoration: none;
+}
+
 /* quick navigation */
 
 #quickNav {
@@ -170,11 +259,11 @@ a {
     margin: 1em;
     padding: 1em;
     font-size: 0.6em;
-    color: gray;
+    color: var(--fg-muted);
 }
 
 #footer a {
-    color: #669;
+    color: var(--link-muted);
 }
 
 /* mobile fixes */
diff --git a/src/SMAPI.Web/wwwroot/Content/css/mods.css b/src/SMAPI.Web/wwwroot/Content/css/mods.css
index 62f8aa398..f494f135f 100644
--- a/src/SMAPI.Web/wwwroot/Content/css/mods.css
+++ b/src/SMAPI.Web/wwwroot/Content/css/mods.css
@@ -28,14 +28,14 @@
     margin: 2px;
     display: inline-block;
     border-radius: 3px;
-    color: #000;
-    border-color: #880000;
-    background-color: #fcc;
+    color: var(--fg);
+    border-color: var(--error-fg);
+    background-color: var(--error-bg);
     font-size: 0.9em;
 }
 
 #options #filters span.active {
-    background: #cfc;
+    background: var(--ok-bg);
 }
 
 #mod-count {
@@ -45,7 +45,7 @@
 
 div.error {
     padding: 2em 0;
-    color: red;
+    color: var(--error-fg);
     font-weight: bold;
 }
 
@@ -53,23 +53,23 @@ div.error {
 ** Mod list
 *********/
 table.wikitable {
-    background-color:#f8f9fa;
-    color:#222;
-    border:1px solid #a2a9b1;
-    border-collapse:collapse
+    background-color: var(--surface);
+    color: var(--fg);
+    border: 1px solid var(--surface-border);
+    border-collapse: collapse;
 }
 
 table.wikitable > tr > th,
 table.wikitable > tr > td,
 table.wikitable > * > tr > th,
 table.wikitable > * > tr > td {
-    border:1px solid #a2a9b1;
-    padding:0.2em 0.4em
+    border: 1px solid var(--surface-border);
+    padding: 0.2em 0.4em;
 }
 
 table.wikitable > tr > th,
 table.wikitable > * > tr > th {
-    background-color:#eaecf0;
+    background-color: var(--surface-alt);
 }
 
 table.wikitable > caption {
@@ -118,21 +118,21 @@ table.wikitable > caption {
 
 #mod-list tr[data-status="ok"],
 #mod-list tr[data-status="optional"] {
-    background: #BFB;
+    background: var(--ok-bg);
 }
 
 #mod-list tr[data-status="workaround"],
 #mod-list tr[data-status="unofficial"] {
-    background: #FFFEC6;
+    background: var(--warn-bg);
 }
 
 #mod-list tr[data-status="broken"] {
-    background: #FBB;
+    background: var(--error-bg);
 }
 
 #mod-list tr[data-status="obsolete"],
 #mod-list tr[data-status="abandoned"] {
-    background: #BBB;
+    background: var(--neutral-bg);
     opacity: 0.7;
 }