diff --git a/packages/review-editor/components/ReviewHeaderMenu.tsx b/packages/review-editor/components/ReviewHeaderMenu.tsx
index 35dcf5f96..585ab21d0 100644
--- a/packages/review-editor/components/ReviewHeaderMenu.tsx
+++ b/packages/review-editor/components/ReviewHeaderMenu.tsx
@@ -1,4 +1,4 @@
-import React, { useMemo } from 'react';
+import React from 'react';
 import {
   ActionMenu,
   ActionMenuDivider,
@@ -6,6 +6,8 @@ import {
   ActionMenuSectionLabel,
 } from '@plannotator/ui/components/ActionMenu';
 import { useTheme } from '@plannotator/ui/components/ThemeProvider';
+import { SunIcon, MoonIcon, SystemIcon } from '@plannotator/ui/components/icons/themeIcons';
+import { getUnsupportedMode } from '@plannotator/ui/utils/themeRegistry';
 import { MenuVersionSection } from '@plannotator/ui/components/MenuVersionSection';
 import { TextShimmer } from '@plannotator/ui/components/TextShimmer';
 import { modKey } from '@plannotator/ui/utils/platform';
@@ -37,10 +39,12 @@ export const ReviewHeaderMenu: React.FC<ReviewHeaderMenuProps> = ({
   origin,
   isWSL = false,
 }) => {
-  const { theme, resolvedMode, setTheme } = useTheme();
-  const activeTheme = useMemo<'light' | 'dark'>(() => {
-    return theme === 'system' ? resolvedMode : theme;
-  }, [resolvedMode, theme]);
+  const { theme, setTheme, colorTheme } = useTheme();
+  const unsupportedMode = getUnsupportedMode(colorTheme);
+  // When the palette can't render the stored mode, the UI is forced to the
+  // opposite — highlight the mode actually shown instead of the dead one.
+  const highlightedTheme =
+    theme === unsupportedMode ? (unsupportedMode === 'light' ? 'dark' : 'light') : theme;
 
   const showUpdateDot = !!updateInfo?.updateAvailable && !updateInfo.dismissed;
 
@@ -80,20 +84,24 @@ export const ReviewHeaderMenu: React.FC<ReviewHeaderMenuProps> = ({
           <div className="px-3 py-2 space-y-1.5">
             <ActionMenuSectionLabel>Theme</ActionMenuSectionLabel>
             <div className="flex items-center gap-1 rounded-lg bg-muted/50 p-0.5">
-              {(['light', 'dark'] as const).map((mode) => (
+              {(['light', 'dark', 'system'] as const).map((mode) => (
                 <button
                   key={mode}
+                  disabled={mode === unsupportedMode}
+                  title={mode === unsupportedMode ? 'Not supported by the current color theme' : undefined}
                   onClick={() => {
                     closeMenu();
                     setTheme(mode);
                   }}
                   className={`flex flex-1 items-center justify-center gap-1.5 rounded-md px-2 py-1 text-[11px] font-medium transition-colors ${
-                    activeTheme === mode
-                      ? 'bg-background text-foreground shadow-sm'
-                      : 'text-muted-foreground hover:text-foreground'
+                    mode === unsupportedMode
+                      ? 'text-muted-foreground opacity-40 cursor-not-allowed'
+                      : highlightedTheme === mode
+                        ? 'bg-background text-foreground shadow-sm'
+                        : 'text-muted-foreground hover:text-foreground'
                   }`}
                 >
-                  {mode === 'light' ? <SunIcon /> : <MoonIcon />}
+                  {mode === 'light' ? <SunIcon /> : mode === 'dark' ? <MoonIcon /> : <SystemIcon />}
                   <span className="capitalize">{mode}</span>
                 </button>
               ))}
@@ -181,19 +189,6 @@ const ExportIcon = () => (
   </svg>
 );
 
-const SunIcon = () => (
-  <svg className="w-3.5 h-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
-    <path strokeLinecap="round" strokeLinejoin="round" d="M12 3v2.25M12 18.75V21M3 12h2.25M18.75 12H21M5.636 5.636l1.591 1.591M16.773 16.773l1.591 1.591M5.636 18.364l1.591-1.591M16.773 7.227l1.591-1.591" />
-    <circle cx="12" cy="12" r="3.25" />
-  </svg>
-);
-
-const MoonIcon = () => (
-  <svg className="w-3.5 h-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
-    <path strokeLinecap="round" strokeLinejoin="round" d="M21 12.79A9 9 0 1111.21 3c-.18.57-.21 1.19-.21 1.82A8 8 0 0019.18 13c.63 0 1.25-.03 1.82-.21z" />
-  </svg>
-);
-
 const FileTreeMenuIcon = () => (
   <svg className="w-3.5 h-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
     <path strokeLinecap="round" strokeLinejoin="round" d="M3 7v10a2 2 0 002 2h14a2 2 0 002-2V9a2 2 0 00-2-2h-6l-2-2H5a2 2 0 00-2 2z" />
diff --git a/packages/ui/components/ActionMenu.tsx b/packages/ui/components/ActionMenu.tsx
index 3ca1f8883..fd6eb26ad 100644
--- a/packages/ui/components/ActionMenu.tsx
+++ b/packages/ui/components/ActionMenu.tsx
@@ -50,7 +50,7 @@ export const ActionMenu: React.FC<ActionMenuProps> = ({
       })}
 
       {isOpen && (
-        <div className={panelClassName ?? 'absolute top-full right-0 mt-1 w-56 rounded-lg border border-border bg-popover py-1 shadow-xl z-[70]'}>
+        <div className={panelClassName ?? 'absolute top-full right-0 mt-1 w-64 rounded-lg border border-border bg-popover py-1 shadow-xl z-[70]'}>
           {children({ closeMenu: () => setIsOpen(false) })}
         </div>
       )}
diff --git a/packages/ui/components/PlanHeaderMenu.tsx b/packages/ui/components/PlanHeaderMenu.tsx
index d5243b36e..940550207 100644
--- a/packages/ui/components/PlanHeaderMenu.tsx
+++ b/packages/ui/components/PlanHeaderMenu.tsx
@@ -7,6 +7,7 @@ import {
 } from './ActionMenu';
 import { useTheme } from './ThemeProvider';
 import { SunIcon, MoonIcon, SystemIcon } from './icons/themeIcons';
+import { getUnsupportedMode } from '../utils/themeRegistry';
 import { ReviewAgentsIcon } from './ReviewAgentsIcon';
 import { MenuVersionSection } from './MenuVersionSection';
 import { TextShimmer } from './TextShimmer';
@@ -58,7 +59,12 @@ export const PlanHeaderMenu: React.FC<PlanHeaderMenuProps> = ({
   bearConfigured,
   octarineConfigured,
 }) => {
-  const { theme, setTheme } = useTheme();
+  const { theme, setTheme, colorTheme } = useTheme();
+  const unsupportedMode = getUnsupportedMode(colorTheme);
+  // When the palette can't render the stored mode, the UI is forced to the
+  // opposite — highlight the mode actually shown instead of the dead one.
+  const highlightedTheme =
+    theme === unsupportedMode ? (unsupportedMode === 'light' ? 'dark' : 'light') : theme;
 
   const showUpdateDot = !!updateInfo?.updateAvailable && !updateInfo.dismissed;
 
@@ -104,14 +110,18 @@ export const PlanHeaderMenu: React.FC<PlanHeaderMenuProps> = ({
               {(['light', 'dark', 'system'] as const).map((mode) => (
                 <button
                   key={mode}
+                  disabled={mode === unsupportedMode}
+                  title={mode === unsupportedMode ? 'Not supported by the current color theme' : undefined}
                   onClick={() => {
                     closeMenu();
                     setTheme(mode);
                   }}
                   className={`flex flex-1 items-center justify-center gap-1.5 rounded-md px-2 py-1 text-[11px] font-medium transition-colors ${
-                    theme === mode
-                      ? 'bg-background text-foreground shadow-sm'
-                      : 'text-muted-foreground hover:text-foreground'
+                    mode === unsupportedMode
+                      ? 'text-muted-foreground opacity-40 cursor-not-allowed'
+                      : highlightedTheme === mode
+                        ? 'bg-background text-foreground shadow-sm'
+                        : 'text-muted-foreground hover:text-foreground'
                   }`}
                 >
                   {mode === 'light' ? <SunIcon /> : mode === 'dark' ? <MoonIcon /> : <SystemIcon />}
diff --git a/packages/ui/utils/themeRegistry.ts b/packages/ui/utils/themeRegistry.ts
index a9da074a3..0941821dd 100644
--- a/packages/ui/utils/themeRegistry.ts
+++ b/packages/ui/utils/themeRegistry.ts
@@ -565,3 +565,16 @@ export const BUILT_IN_THEMES: ThemeInfo[] = [
     },
   },
 ];
+
+/**
+ * The mode a color theme cannot render, or null if it supports both.
+ * Dark-only palettes silently ignore light mode (and vice versa) in
+ * ThemeProvider — callers use this to disable the incompatible mode
+ * button instead of letting the toggle no-op.
+ */
+export function getUnsupportedMode(themeId: string): 'light' | 'dark' | null {
+  const info = BUILT_IN_THEMES.find(t => t.id === themeId);
+  if (info?.modeSupport === 'dark-only') return 'light';
+  if (info?.modeSupport === 'light-only') return 'dark';
+  return null;
+}