Implement UI theme colors.

web/react/components/user_settings/user_settings_developer.jsx

@@ -64,7 +64,7 @@ export default class DeveloperTab extends React.Component {
                         data-dismiss='modal'
                         aria-label='Close'
                     >
-                        <span aria-hidden='true'>{'x'}</span>
+                        <span aria-hidden='true'>{'×'}</span>
                     </button>
                     <h4
                         className='modal-title'