Bug 1832549 - Make menuitem border radius and panel / menupopup padding dynamic by referencing --panel-border-radius. r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D177762

toolkit/themes/linux/global/menu.css

@@ -20,7 +20,7 @@ menubar {
 menu,
 menuitem,
 menucaption {
-  border-radius: 4px;
+  border-radius: calc(var(--panel-border-radius) / 2);
   align-items: center;
   flex-shrink: 0;
   padding: 4px 6px;

toolkit/themes/linux/global/popup.css

@@ -10,7 +10,7 @@
 menupopup,
 panel {
   min-width: 1px;
-  --panel-padding-block: 3px;
+  --panel-padding-block: max(0px, var(--panel-border-radius) / 2 - 1px);
   --panel-padding: var(--panel-padding-block) 0;
   --panel-color: MenuText;
   --panel-background: Menu;