Setting a gradient for the horizontal tabline doesn't work

[jonathansharpe]

Describe the bug
Setting the "--tl-tab-background-gradient" value to something has no visual effect. I could be doing it wrong, but also the wiki doesn't give super specific instructions on how to do it. I've tried linear-gradient, a color, "--dark-4", and none of those work.

Firefox Version:
Version 93.0
Mozilla Firefox for Manjaro Linux