removed unused CSS

- rm readable.css (not needed in new theme)
- link to color guide

Author: orbeckst

docs/_static/custom.css

@@ -1,6 +1,6 @@
-/* override css for readable.css */
+/* custom MDAnalysis styling (User Guide theme) */
 
-/*  styles/fonts to match http://mdanalysis.org (see public/css) */
+/* See https://github.com/MDAnalysis/mdanalysis/wiki/MDAnalysis-theme-colours */
 /* MDAnalysis orange: #FF9200 */
 /* MDAnalysis gray: #808080 */
 /* MDAnalysis white: #FFFFFF */
@@ -203,4 +203,4 @@ dl.field-list > dt::after {
 
 .rst-content code:not(.xref).literal {
     color: #ca6500;
-}
+}