docs(style): update dark background color

Author: ace-of-aces

docs/.vitepress/theme/custom.css

@@ -11,4 +11,5 @@
     --vp-c-brand-2: #cbc4c4;
     --vp-c-brand-3: #8a8585;
     --vp-c-brand-soft: rgba(115, 110, 110, 0.14);
+    --vp-c-bg: #121212;
 }