fix: match background in dark mode overscroll

In macOS and iOS devices you can scroll past the body. When this happens
in dark mode, the css variables are only changed from the `body`, and
the ones used in `html` are not changed. This causes the overscroll
background to be displayed as the light mode color.

styles/index.css

@@ -12,9 +12,12 @@ body {
   flex: 1;
 
   font-size: var(--font-size-normal);
+  overflow-y: overlay;
+}
+
+body {
   color: var(--gray900);
   background: var(--gray75);
-  overflow-y: overlay;
 }
 
 .zh-CN {