diff --git a/R/helpers-gg.R b/R/helpers-gg.R
index 953a51d4..5aeaf0cc 100644
--- a/R/helpers-gg.R
+++ b/R/helpers-gg.R
@@ -10,8 +10,9 @@
 #' @examples
 #' # Draw a vertical line at zero (or do nothing)
 #' xs <- -2:2
+#' style <- annotation_style()
 #' maybe_vertical_line <- if (0 > min(xs) && 0 < max(xs)) {
-#'   vline_0(color = "gray90", linewidth = 0.5)
+#'   vline_0(color = style$color, linewidth = style$linewidth)
 #' } else {
 #'   geom_ignore()
 #' }
@@ -75,6 +76,29 @@ force_x_axis_in_facets <- function() {
   )
 }
 
+# Derive annotation line aesthetics from the active theme's gridlines.
+# When the theme has visible gridlines, the reference line inherits their
+# color at twice the major gridline width. When gridlines are blank (e.g.
+# bayesplot's default theme), falls back to a light gray.
+annotation_style <- function() {
+  thm <- bayesplot_theme_get()
+  grid <- calc_element("panel.grid.major", thm)
+  if (inherits(grid, "element_blank") || is.null(grid)) {
+    return(list(color = "gray90", linewidth = 0.5))
+  }
+  minor <- calc_element("panel.grid.minor", thm)
+  minor_lw <- if (!inherits(minor, "element_blank") && !is.null(minor$linewidth)) {
+    minor$linewidth
+  } else {
+    0.125
+  }
+  major_lw <- grid$linewidth %||% (minor_lw * 2)
+  list(
+    color = grid$colour %||% "gray90",
+    linewidth = major_lw * 2
+  )
+}
+
 no_legend_spacing <- function() {
   theme(legend.spacing.y = unit(0, "cm"))
 }
diff --git a/R/mcmc-diagnostics.R b/R/mcmc-diagnostics.R
index 2727928f..532dd076 100644
--- a/R/mcmc-diagnostics.R
+++ b/R/mcmc-diagnostics.R
@@ -154,8 +154,9 @@ mcmc_rhat <- function(rhat, ..., size = NULL) {
       bayesplot_theme_get()
 
   if (min(data$value) < 1) {
+    ref_style <- annotation_style()
     graph <- graph +
-      vline_at(1, color = "gray", linewidth = 1)
+      vline_at(1, color = ref_style$color, linewidth = ref_style$linewidth)
   }
 
   brks <- set_rhat_breaks(data$value)
diff --git a/R/mcmc-intervals.R b/R/mcmc-intervals.R
index 1ab9298a..3d6c87ee 100644
--- a/R/mcmc-intervals.R
+++ b/R/mcmc-intervals.R
@@ -222,8 +222,9 @@ mcmc_intervals <- function(x,
   x_lim[2] <- x_lim[2] + 0.05 * x_range
 
   # faint vertical line at zero if zero is within x_lim
+  ref_style <- annotation_style()
   layer_vertical_line <- if (0 > x_lim[1] && 0 < x_lim[2]) {
-    vline_0(color = "gray90", linewidth = 0.5)
+    vline_0(color = ref_style$color, linewidth = ref_style$linewidth)
   } else {
     geom_ignore()
   }
@@ -338,8 +339,9 @@ mcmc_areas <- function(x,
   x_lim[1] <- x_lim[1] - 0.05 * x_range
   x_lim[2] <- x_lim[2] + 0.05 * x_range
 
+  ref_style <- annotation_style()
   layer_vertical_line <- if (0 > x_lim[1] && 0 < x_lim[2]) {
-    vline_0(color = "gray90", linewidth = 0.5)
+    vline_0(color = ref_style$color, linewidth = ref_style$linewidth)
   } else {
     geom_ignore()
   }
@@ -501,8 +503,9 @@ mcmc_areas_ridges <- function(x,
   x_lim[1] <- x_lim[1] - 0.05 * x_range
   x_lim[2] <- x_lim[2] + 0.05 * x_range
 
+  ref_style <- annotation_style()
   layer_vertical_line <- if (0 > x_lim[1] && 0 < x_lim[2]) {
-    vline_0(color = "gray90", linewidth = 0.5)
+    vline_0(color = ref_style$color, linewidth = ref_style$linewidth)
   } else {
     geom_ignore()
   }