There's a one-pixel line in Firefox Color!

[Sotono]

Hello! I've recently started using Firefox again and adjusted my theme using Firefox Color.
I created a theme similar to Chrome's Kate Spade, but I'm bothered by a one-pixel line under the bookmark bar where the background is applied.
I apologize if this has been addressed before, but I would appreciate it if you knew how to fix this.
Thank you.

[Sotono · Accepted solution]

OK, I've figured out that I can solve this issue by adjusting the settings here!
It turned out not to be a problem at all. Thank you!