feat: default browser to dark mode

Set colorScheme: 'dark' as the default context option so websites
render in dark mode by default when supported.

src/config.ts

@@ -68,6 +68,7 @@ const defaultConfig: FullConfig = {
     },
     contextOptions: {
       viewport: null,
+      colorScheme: 'dark',
     },
   },
   network: {