Implement settings close button hover

Closes #439

src/browser/components/SettingsPage.jsx

@@ -451,6 +451,7 @@ const SettingsPage = createReactClass({
             <h1 style={settingsPage.heading}>{'Settings'}</h1>
             <Button
               id='btnClose'
+              className='CloseButton'
               bsStyle='link'
               style={settingsPage.close}
               onClick={this.handleCancel}

src/browser/css/settings.css

@@ -1,3 +1,6 @@
+.CloseButton:hover span {
+  color: #333;
+}
 
 .IndicatorContainer {
   position: absolute;