Follow core base font-size in Vector

Following core base font-size of `0.875em` in Vector to align with
other elements on the page.

Bug: T175295
Change-Id: Id9c8652820641d6fa1c6b1dd06247967ba649f25

extension.json

@@ -160,7 +160,8 @@
 			"scripts": "js/ext.uls.common.js",
 			"styles": "css/ext.uls.less",
 			"skinStyles": {
-				"monobook": "css/ext.uls-monobook.css"
+				"monobook": "css/ext.uls-monobook.css",
+				"vector": "css/ext.uls-vector.less"
 			},
 			"dependencies": [
 				"mediawiki.api",

resources/css/ext.uls-vector.less

@@ -0,0 +1,3 @@
+.uls-menu {
+	font-size: 0.875em; // not inherited from `.mw-body-content`, as we insert at `body`
+}