Bar Focus - how to change style of collapser icon?

My Mautic version is: 4.1.0

My problem is: I want to change the style of collapser of the Focus Bar element. I would like to change it to X sign in upper-right corner of the bar.

I tried to use new “Custom CSS” field in 4.1.0 to change the style of “mf-bar-collapser” class. However I see no changes.

Could you please point me to the right direction, how should I do it?