Revert "Temp: test @supports query for CSS custom props"

This reverts commit df64807384.

tildes/scss/themes/_theme_mixins.scss

@@ -141,13 +141,6 @@
   $theme: init-theme($selected-theme);
   $is-light: is-color-bright(map-get($theme, "background-primary"));
 
-  // Temp: test whether this @supports query works for some users with older browsers
-  @supports not (--test: green) {
-    #sidebar {
-      background-color: #fff;
-    }
-  }
-
   // When creating CSS custom properties and using any of Sass' capabilities
   // you'll have to interpolate it with the Sass syntax `#{...}` as seen below.
   --alert: #{map-get($theme, "alert")};