Disable language button background color on modern Vector

Limit this style to sidebar buttons. When skins are not placing it
inside the sidebar (e.g. modern Vector), we should defer to them for
all styling matters. This rule clashes with modern Vector's mw-ui-button
class.

Bug: T296323
Change-Id: I16bdbf8882e37b8727fbf693c904b9c9d1d8820c

resources/css/ext.uls.compactlinks.less

@@ -21,11 +21,11 @@
 	text-align: left;
 	cursor: pointer;
 }
-/* stylelint-enable selector-max-id */
 
-.mw-interlanguage-selector.selector-open {
+#p-lang .mw-interlanguage-selector.selector-open {
 	background-color: #c8ccd1;
 }
+/* stylelint-enable selector-max-id */
 
 /*
  * Add caret to the ULS dialog when using the cog in the sidebar.