docs: fix search color (zarr-developers#3777)

Author: maxrjones

docs/overrides/stylesheets/extra.css

@@ -56,11 +56,6 @@
 .md-header .md-search__input {
   background-color: rgba(255, 255, 255, 0.15);
   border: 1px solid rgba(255, 255, 255, 0.2);
-  color: white;
-}
-
-.md-header .md-search__input::placeholder {
-  color: rgba(255, 255, 255, 0.7);
 }
 
 /* Navigation tabs */