Fix branch selector UI (#30803) (#30819)

Backport #30803 by wxiaoguang

Fix  #30802

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/css/base.css

@@ -875,6 +875,7 @@ input:-webkit-autofill:active,
 
 .ui.dropdown .scrolling.menu {
   border-color: var(--color-secondary);
+  border-radius: 0 0 var(--border-radius) var(--border-radius) !important;
 }
 
 .color-preview {