Short description

The colors for the Plain style used to inherit the browser theme, which allowed it to match the GTK theme on Linux by default. But for some reason, they are now identical to the Plain Dark theme, which forces some color choices. Many other styles have this behavior. High Contrast does not, but it also has a really wonky background color.

Expected result

Tree Style Tab blends in with the GTK theme by default and looks like a part of Firefox's UI.

Actual result

It sticks out like a sore thumb. It looks like this item in the release notes has something to do with it:
"Always apply the "Photon" color scheme on Linux environments."
Why was this done? It seems counter to your intent as stated in issue 2662.

Environment

• Platform (OS): Xubuntu 18.04
• Version of Firefox: 79.0
• Version (or revision) of Tree Style Tab: 3.5.15