Enable Stylelint color-hex-length (#34310)

app/javascript/styles/mastodon-light/variables.scss

@@ -5,8 +5,8 @@
   $lighten-multiplier: -1
 );
 
-$black: #000000; // Black
-$white: #ffffff; // White
+$black: #000; // Black
+$white: #fff; // White
 $blurple-500: #6364ff; // Brand purple
 $grey-600: hsl(240deg, 8%, 33%); // Trout
 $grey-100: hsl(240deg, 51%, 90%); // Topaz