I had the same “issue” and by checking the code for drawing a button background I found it uses ComboBox::outlineColourId colour to draw the outline.
So, you just need to set that colour to change the button outline.
Wow, I checked the source because I couldn’t believe it:
I think that should be at least documented, since TextButton and ComboBox are competely unrelated.
But IMHO it should get it’s own colourId TextButton::outlineColourId, even though that changes behaviour. It’s rather a bug to start with.