gui: Adjust border-radius on accordion title buttons (fixes #3299)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3320

gui/default/assets/css/overrides.css

@@ -191,6 +191,7 @@ button.panel-heading {
     border-top-width: 0;
     border-left-width: 0;
     border-right-width: 0;
+    border-radius: 0 !important;
 }
 
 .panel-heading .panel-title-text {