diff --git a/src/extension-api/types.ts b/src/extension-api/types.ts
index e0bebe63..ecab2f15 100644
--- a/src/extension-api/types.ts
+++ b/src/extension-api/types.ts
@@ -1489,6 +1489,14 @@ export interface HunkExtensionAPI {
   readonly config: Record<string, unknown>;
   /** Record a diagnostic line; collected per extension instead of written to the terminal. */
   log(message: string): void;
+  /**
+   * Switch the active theme programmatically.
+   *
+   * Valid at any time — during the factory, from a lifecycle handler, or from
+   * a command handler. The theme id must match a built-in, config-defined, or
+   * extension-registered theme.
+   */
+  setTheme(themeId: string): void;
 }
 
 /** Default export every extension entry file must provide. */
diff --git a/src/extensions/runExtension.ts b/src/extensions/runExtension.ts
index 3a20c7be..d3f01df5 100644
--- a/src/extensions/runExtension.ts
+++ b/src/extensions/runExtension.ts
@@ -448,6 +448,12 @@ export function createExtensionApi(
       // Logs are collected rather than printed: the TUI owns the terminal.
       registry.logs.push({ extensionId: metadata.id, message: String(message) });
     },
+    setTheme(themeId: string) {
+      // Runtime action, not a registration — no assertOpen guard.
+      // The callback is bound after the UI mounts; before that, calls are a
+      // silent no-op since there is nothing to switch yet.
+      registry.setTheme?.(themeId);
+    },
   };
 
   return {
diff --git a/src/extensions/types.ts b/src/extensions/types.ts
index 648f823d..fe37e1d2 100644
--- a/src/extensions/types.ts
+++ b/src/extensions/types.ts
@@ -187,6 +187,8 @@ export interface ExtensionRegistry {
   eventBusPhase: "loading" | "ready" | "closed";
   /** Bound after loading so hunk.events.emit can dispatch at runtime. */
   emitCustomEvent?: (event: string, payload: unknown) => void;
+  /** Bound after the UI mounts so hunk.setTheme can switch themes at runtime. */
+  setTheme?: (themeId: string) => void;
   logs: ExtensionLogEntry[];
 }
 
diff --git a/src/ui/App.tsx b/src/ui/App.tsx
index d8d549d3..4771dcbb 100644
--- a/src/ui/App.tsx
+++ b/src/ui/App.tsx
@@ -1182,6 +1182,11 @@ export function App({
     },
     [showTransientNotice, themeOptions],
   );
+  // Wire hunk.setTheme into the React theme state so extensions can switch
+  // themes programmatically at runtime.
+  if (extensions) {
+    extensions.registry.setTheme = selectTheme;
+  }
 
   /** Open the keyboard-driven theme selector with the current theme highlighted. */
   const openThemeSelector = useCallback(() => {