Does anyone know what is the GTK3 CSS class for the "brush selection box"?

I think it applies to any GTK3 version of MyPaint.

I’m trying to adjust a GTK3 theme to be more compact, my main focus is to use it with MyPaint itself.

Right now the most promising one seems to be Numix. The padding around the sliders is already minimal.

The biggest problem is the indication of which brush is selected. While some themes will draw a box of some color around the selected brush, on Numix it can look “even less selected” as instead there’s some minimal white beveling thing they do. I’d like to reproduce a more clear box/border around the selected brush icon, that I believe I could do, if only I knew what the brush selection box itself is called.

GTK_DEBUG=interactive wasn’t really helpful, maybe I just don’t quite get where to look. It’s definitely not as obvious as it would be on Google Chrome or Firefox “inspector.”

Out of trial and error, copying and pasting bits from the CSS of a theme that had this part right, I’ve managed to apparently obtain the result I wanted by adding this bit to the end of the oomox-tweaked Numix’ CSS:

    background-color: #64a2df;
    color: @fg_color;

It’s probably defining in excess of what I really need, maybe it has unintended collateral effects, but I didn’t notice anything weird so far.