MAGETWO-32196: Visual style for 'skip to content' attribute

Author: Natalia Momotenko

app/design/frontend/Magento/blank/Magento_Theme/web/css/source/module.less

@@ -87,6 +87,7 @@ body {
     }
     &:focus {
         position: absolute;
+        z-index: 15;
         box-sizing: border-box;
         width: 100%;
         left: 0;

app/design/frontend/Magento/luma/Magento_Theme/web/css/source/module.less

@@ -159,6 +159,7 @@
         }
         &:focus {
             position: absolute;
+            z-index: 15;
             box-sizing: border-box;
             width: 100%;
             left: 0;