Change mce-active class to is-active.

Author: ellatrix

shared/tinymce/toolbar.js

@@ -49,6 +49,9 @@
 			} );
 
 			return this;
+		},
+		active: function( isActive ) {
+			this.$( this.getEl() ).toggleClass( 'is-active', isActive );
 		}
 	} );
 

tinymce-single/tinymce/block.css

@@ -71,7 +71,7 @@
 	background-color: #f0f2f4;
 }
 
-.mce-inline-toolbar-grp .mce-btn.mce-active,
+.mce-inline-toolbar-grp .mce-btn.is-active,
 .mce-inline-toolbar-grp .mce-btn:active {
 	background-color: #e0e5e9;
 }