Merge style sheets for light and dark themes into a single resource

This will avoid having inline style, which is annoying for stricter CSP.
It should improve performances, as there is only a single file to load.
As a bonus, I also included resource fingerprinting, which will avoid breaking
the cache when it's not needed, and adds subresource integrity checksum.

assets/css/dark.css

@@ -0,0 +1,24 @@
+/* dark.css */ 
+--font-color: #eee;
+--bg-color: #212121;
+
+--link-color:#599ada;
+--link-state-color:#ff5858;
+--link-state-border-color: rgba(238, 54, 54, 0.5);
+
+--thead-bg-color: #343a40;
+--table-border-color: lightgrey;
+
+--pre-color: #333;
+--pre-bg-color: #f1f1f1;
+
+--bq-color: #ccc;
+--hr-color: #333;
+
+--pagination-bg-color: #373737;
+--pagination-link-color: #b6b6b6;
+
+--post-info-color: grey;
+
+--switcher-color: #333;
+--switcher-bg-color: #fff;