Window control buttons on the Mate Desktop

[Fausto-Korpsvart]

Greetings, @vinceliuice.

I was checking the Theme and came across this error, I don't know enough to know where the problem might be coming from, the problem only affects the window control buttons on the Mate desktop.

Here is a proof of what I mean.

MateDesktop.mp4

Any hints on how to fix it?

Thanks for your great Themes, they are inspiring.