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/readthrough/readthrough_list.html

@@ -3,7 +3,7 @@
 {% load tz %}
 {% load utilities %}
 <div class="content">
-    <div class="box is-shadowless has-background-white-bis">
+    <div class="box is-shadowless has-background-tertiary">
         <div class="columns">
             <div class="column">
                 {% trans "Progress Updates:" %}