docs: fix mobile menu background on light mode landing page (#18633) (#1719)

Co-authored-by: 翠 / green <[email protected]>

Author: kiaking

.vitepress/theme/styles/landing.css

@@ -3,17 +3,15 @@
 /* /////////////////////// */
 
 html:has(.landing) {
+  --vp-c-bg: #101010;
+
   background-color: #101010;
 
   body {
     background-color: #101010;
   }
 }
 
-html.dark:has(.landing) {
-  --vp-c-bg: #101010;
-}
-
 .landing {
   overflow-x: hidden;
   background-color: #101010;