normalize hex color to lowercase

Author: Fil

docs/.vitepress/theme/custom.css

@@ -10,7 +10,7 @@
   --vp-c-purple-2: #9a5ae8;
   --vp-c-purple-3: #884ad6;
   --vp-c-purple-soft: rgba(155, 91, 233, 0.16);
-  --vp-c-text-1: #F5F5F5;
+  --vp-c-text-1: #f5f5f5;
 }
 
 :root {