allow change nextThemes.forcedTheme prop (#4189)

Author: dimaMachina

.changeset/early-fishes-carry.md

@@ -0,0 +1,5 @@
+---
+"nextra-theme-docs": patch
+---
+
+allow change `nextThemes.forcedTheme` prop

packages/nextra-theme-docs/src/layout.tsx

@@ -57,6 +57,7 @@ const theme = z.strictObject({
         .default('class'),
       defaultTheme: z.string().optional(),
       disableTransitionOnChange: z.boolean().default(true),
+      forcedTheme: z.string().optional(),
       storageKey: z.string().optional()
     })
     .default({}),