Refresh Jellyfin screenshots and setup docs

docs/themes/jellyfin.md

@@ -9,25 +9,42 @@ Custom [{{ page.title.split()[0] }}](https://github.com/jellyfin/jellyfin) CSS
 
 ## 🛠️ Installation
 
-### Setup
+Choose one method below. Use subfiltering if you want the dashboard
+themed too.
 
-Go to `Dashboard` -> `General` and scroll down to `Branding`
+### Built-in Custom CSS
 
-In the custom CSS input field add:
+These screenshots show Jellyfin 12.0.
+
+Open **Dashboard > Branding** and add this to **Custom CSS code**, replacing
+`<THEME>` with a [theme option](/theme-options/). Save your changes.
 
 ```css
 @import url("https://theme-park.dev/css/base/jellyfin/<THEME>.css");
 ```
 
-and hit save.
-
-i.e.
+For example, Nord:
 
 ```css
 @import url("https://theme-park.dev/css/base/jellyfin/nord.css");
 ```
 
-<img src="/site_assets/{{ page.title.split()[0].lower() }}/example.png"></img>
+### Subfiltering, including the dashboard
+
+Jellyfin 12 does not load its Custom CSS setting in the admin dashboard.
+To theme those pages too, use [subfiltering](/setup/#subfiltering) instead of
+the built-in method. Remove any theme.park import from **Custom CSS code**
+and save before switching.
+
+Follow the setup guide for your reverse proxy and inject this stylesheet into
+the web client HTML, replacing `<THEME>` with your selected theme option:
+
+```text
+https://theme-park.dev/css/base/jellyfin/<THEME>.css
+```
+
+Keep injection out of API, media, and WebSocket responses. Subfiltering themes
+both regular pages and the dashboard without a second import in Jellyfin.
 
 {% set addons = extra.addons %}
 {% set title = page.title.split()[0].lower() %}