fix(core): incorrect z-index for the inner header container

Also fix the undefined LESS variable in the Echo skinstyle

resources/skins.citizen.styles/Header.less

@@ -91,6 +91,7 @@
 	}
 
 	&__inner {
+		z-index: -1; // Inner element should be behind menu and search
 		display: flex;
 		min-width: 0;
 		flex-grow: 1;

skinStyles/extensions/Echo/ext.echo.ui.less

@@ -8,6 +8,8 @@
  * Date: 2021-08-11
 */
 
+@import '../../../resources/variables.less';
+
 /* mw.echo.ui.mobile.less */
 @media all and ( max-width: 720px ) {
 	.mw-echo-ui-overlay {