fix(vscode-theming): refine background CSS variable mapping

Author: Ivanruii

apps/web/src/core/vscode/use-vscode-theme.hook.ts

@@ -7,7 +7,7 @@ import {
 import { useEffect } from 'react';
 
 const CSS_VAR_MAP: Record<keyof ThemePayload, readonly string[]> = {
-  background: ['--primary-100', '--primary-500', '--primary-200'],
+  background: ['--primary-100', '--primary-200'],
   backgroundSecondary: ['--pure-white'],
   foreground: ['--primary-700'],
 };