Styling OverlayDialogs - how?

GIMP 3.0.6, dark theme, Linux

I need to put a visible border around the overlay dialogs ( like uniform transform tool etc).
As it is now I cannot recognize its borders on the more or less equal coloured background.

So I’ve found I can use this:

GimpOverlayDialog {border: 1px solid white;}

But where do I put it?

I’ve tried a gimp.css file in my ~/.config/GIMP/3.0 folder which doesn’t seem to work.
Also tried in the themes folder, didn’t work either.

So please help.

Yet this is the right method to add custom CSS but indeed even though it works in the GtkInspector, this same CSS doesn’t work when added in your custom gimp.css (but I tried other random changes like * { color:red } and they worked so it’s not the gimp.css loading which is broken).

I thought maybe it was something about the order of imports, but it doesn’t seem to to be it either (currently it’s imported last, which I assume means that any rules in this file should override previous ones? @CmykStudent?). I did still try reverting import orders, and as thought, it didn’t fix anything.

Maybe we hardcode something in the code and this happens after CSS loads (unlike when we add it to GtkInspector)?

@CmykStudent Any idea?