Are you talking about the new Quantum theme? https://addons.mozilla.org/en-US/firefox/addon/classic-theme-restorer-1-7-8

or the original Legacy extension? (Not compatible with Quantum Firefox) ?

Have you contacted their support?

Hi Bob, are you talking this issue: when using an add-on theme or the Firefox Alpenglow theme, the background image or background color has a haze or fog over it on the main toolbar compared with the tabs bar. This is intended to make it earlier to pick out buttons against a busy background.

Extensions cannot remove that any more -- any advice for extensions that ran on Firefox 56 and earlier related to the toolbar area is nearly all obsolete in Firefox 57+.

You can, however, remove it using custom style rules in a userChrome.css file. That's a little bit involved, so before deluging you with further information, is that what you meant, or something else?