Dynamic window margins

[barbiefan]

Hello!
I have margin 10px for the whole bar and I want to set it to 0 when there is solo window with specific class name
I know how to "detect" this state:
window#waybar.chromium { padding: 0; margin: 0; }
but it doesn't change anything (I think it applies to the wrong css class or something)

I tried to see what's the css class which has this margin but it seems like root window itself is padded:

Is there no way to achieve what I want?