Fix: Theme milestone progress bar statuses not rendered correctly

Fixes Milestones progress bar statuses not rendered correctly after Gitea
1.23.x update. Maps explicit styling values for progress bars to existing colours.
Part of infrastructure/blender-projects-platform#133

public/assets/css/theme-bthree-dark.css

@@ -518,6 +518,12 @@ a.ui.primary.label:hover,
 	 color: #dbdbeb;
 }
 
+/* Progress bar. */
+progress::-moz-progress-bar,
+progress::-webkit-progress-value {
+  background-color: var(--color-primary);
+}
+
 /* Style Overrides. */
 #navbar .dropdown .item {
   justify-content: flex-start !important;