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/settings/reports/report_preview.html

@@ -4,7 +4,7 @@
 {% load utilities %}
 
 {% block card-header %}
-<h2 class="card-header-title has-background-white-ter is-block">
+<h2 class="card-header-title has-background-secondary is-block">
     <a href="{% url 'settings-report' report.id %}">
         {% include "settings/reports/report_header.html" with report=report %}
     </a>