feat(theme): use `@layer __vitepress_base` to wrap the styles in base.css (#4905)

src/client/theme-default/styles/base.css

@@ -1,3 +1,4 @@
+@layer __vitepress_base {
   @media (prefers-reduced-motion: reduce) {
     *,
     ::before,
@@ -249,3 +250,4 @@ mjx-container > svg {
     display: inline-block;
     margin: auto;
   }
+}