"open a new tab" button background now gray

Does the issue occur without any files inside /chrome/ folder?
-> no

Issue description, userChrome.css/userContent.css 'settings' the issue occurs with and steps to reproduce the issue:

I have no idea what .css file causes this. I have no idea what the name of this field would be so I can search for it in the .css files. How would I find out?

"open a new tab" and "list all tabs" icons on tabs toolbar has a gray background, making the icons near impossible to see.

Does the issue occur with the most recent version of CustomCSSforFx (release/test/dev build)?
I did nothing to change anything. This simply started to happen.
I aM USING CSS 4.5.2

Screenshots showing the issue (drag & drop images into this post):


System information

like what? I deleted all the instructions like i was told to.

Windows 10
FF 137.0
css 4.5.2

Start by reproducing the tab-toolbar appearance with the reported Windows 10, Firefox 137.0, and CustomCSSforFx 4.5.2 setup, then inspect the CSS files in the /chrome/ folder, especially userChrome.css and the rules affecting the “open a new tab” and “list all tabs” icons. Done means both icons are visible without the unintended gray background.

Written by the indexing model from the issue text.