Apply user-defined CSS after plugins' JavaScript runs (#3661)

src/main/scala/gitbucket/core/controller/IndexController.scala

@@ -192,6 +192,16 @@ trait IndexControllerBase extends ControllerBase {
     Ok()
   }
 
+  get("/user.css") {
+    context.settings.userDefinedCss match {
+      case Some(css) =>
+        contentType = "text/css"
+        css
+      case None =>
+        NotFound()
+    }
+  }
+
   /**
    * Set account information into HttpSession and redirect.
    */

src/main/twirl/gitbucket/core/main.scala.html

@@ -159,5 +159,10 @@
         @Html(script)
       </script>
     }
+    @if(context.settings.userDefinedCss.isDefined) {
+      <script>
+        $('head').append($('<link rel="stylesheet" type="text/css" href="@context.baseUrl/user.css">'))
+      </script>
+    }
   </body>
 </html>