use variable to set width and height of nav toggle button

Author: mojavelinux

src/css/toolbar.css

@@ -22,9 +22,9 @@
   border: none;
   outline: none;
   line-height: inherit;
-  height: 2.5rem;
   padding: 0;
-  width: 2.5rem;
+  height: var(--toolbar-height);
+  width: var(--toolbar-height);
   margin-right: -0.25rem;
 }