Dark mode: fix contrast of footer highlight

Author: hugovk

python_docs_theme/static/pydoctheme_dark.css

@@ -60,6 +60,10 @@ span.highlighted {
     background-color: #616161;
 }
 
+.footnote:target {
+    background-color: #2c3e50;
+}
+
 /* Below for most things in text */
 
 dl.field-list > dt {