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/light.css

@@ -0,0 +1,26 @@
+:root {
+   /* light.css */ 
+    --font-color: #333;
+    --bg-color: #fff;
+
+    --link-color:#1d60a3;
+    --link-state-color:#a31d1d;
+    --link-state-border-color: rgba(163, 29, 29, .5);
+
+    --thead-bg-color: lightgrey;
+    --table-border-color: lightgrey;
+
+    --pre-color: #333;
+    --pre-bg-color: #f1f1f1;
+
+    --bq-color: #ccc;
+    --hr-color: #ccc;
+
+    --pagination-bg-color: #fafafa;
+    --pagination-link-color: #696969;
+
+    --post-info-color: grey;
+
+    --switcher-color: #fff;
+    --switcher-bg-color: #333; 
+}