Hello Ayman,

The best way to implement custom styling is through the stylesheets and not directly on to the control/icon.

To see if your styling is working well the best thing to do is to run your application and check the application in the browser.

More info can be found here :

https://docs.mendix.com/howto/front-end/customize-styling-new/

or in the learning paths.

Hope this helps,

Good luck!