A part of the styling suddenly disappears

I made and use a theme that changes the colors of the address bar and the highlighted text etc… (this theme) but every now and then (a couple of times a day), that particular bit changes back to the native orange-white (I am on Ubuntu), disabling and enabling the theme again fixes it, for a while.
It seems to ignore the “toolbar_field_focus”, “toolbar_field_highlight”, “toolbar_field_border_focus”, “toolbar_field” and “toolbar_field_text”.

Does anyone know why this could be?

Do you have another extension that modifies parts of the UI via themes API?

I have the Firefox color extension installed but haven’t used it for a few days. (I just removed it to see if the problem disappears)