Fix dark mode link color in mirror-description and other non-repo-list areas

Add a general a:link, a:visited dark mode rule so links anywhere in the
page (e.g. the description block) use the correct color instead of the
browser's default purple.

mirror-file-generator/templates/default/header.html

@@ -94,6 +94,10 @@
                 background-color: #1e1e1e;
                 color: #e0e0e0;
             }
+            a:link,
+            a:visited {
+                color: #e0e0e0;
+            }
             .repo-list a:link,
             .repo-list a:visited {
                 color: #e0e0e0;