Hi Branislav,
You can create different classes for this image or icon in your style sheets and then use the css selector helper widget to apply the specific class that you want.
https://appstore.home.mendix.com/link/app/35039/ITvisors/CSS-selector-helper-widget
Here is a test project on github.
https://github.com/Itvisors/mendix-CssSelectorHelperWidget/tree/master/test
also some documentation on how to get setup with Koala
https://docs.mendix.com/howto/ux/setup-mendix-ui-framework-with-koala
Hope this helps!