fix: ensure popover's button is not visible

The background of the popover button was visible when using a tertiary
button as the popover target

Author: chrisolsen

libs/web-components/src/components/popover/Popover.svelte

@@ -437,6 +437,7 @@
     outline: none;
     border: none;
     padding: 0;
+    background-color: transparent;
     width: inherit;
   }