fix milestone progress bar

In default style this is.`.bg-purple` and is `!important;` so we need to manually override this.

Author: the-j0k3r

github-dark.css

@@ -3111,6 +3111,9 @@
   .discussion-item-icon {
     background: #444 !important;
   }
+  .progress-bar .progress.bg-purple {
+    background-color: #6f42c1 !important;
+  }
   .switcher > span:after, .list-module a:hover, .section-nav .active a,
   #dashboard .filter-bar li a.filter_selected:after, .state-widget .behind .bar,
   .settings-email .label:not(.default), .key, .date:after,