Split css (#1959)

Divides the css into sub-files and normalizes how colors are defined.

Co-authored-by: Joachim <joachim.robert@protonmail.com>

bookwyrm/templates/discover/discover.html

@@ -15,7 +15,7 @@
         </p>
     </header>
 
-    {% with tile_classes="tile is-child box has-background-white-ter is-clipped" %}
+    {% with tile_classes="tile is-child box has-background-secondary is-clipped" %}
     <div class="tile is-ancestor">
         <div class="tile is-6 is-parent">
             <div class="{{ tile_classes }}">