Affected
Ubuntu 20 64bit

not affected
WIndows10, MacOS 10.15

Affected versions
Firefox nightly 90.0a1
Firefox beta 89.0b11

Preconditions
install theme https://addons.mozilla.org/es/firefox/addon/matte-black-red/
or https://addons.mozilla.org/en-US/firefox/addon/dark-theme-for-firefox/?utm_source=addons.mozilla.org&utm_medium=referral&utm_content=search
or https://addons.mozilla.org/en-US/firefox/addon/abyssyellow/?utm_source=addons.mozilla.org&utm_medium=referral&utm_content=search
Proton ON

Steps to reproduce
Launch Firefox and enable the search bar in toolbar ( in hamburger menu>settings>search)
Click on search bar and type anything

Actual result
the separators on searchbar for all the Custom Dark Themes have a black line that is almost not visible

expected result
The separators should be grey, this is the color in release, and also the color in the URLbar with search suggestions

Note this happens on ubuntu only, on mac and windows i saw grey separators

Hi romain, could you please set a priority for bug?
thanks!