Letterboxing not themed #45

When letterboxing is enabled (privacy.resistFingerprinting.enable = true + privacy.resistFingerprinting.letterboxing = true), the added bars are not themed. This can be somewhat fixed by setting the tabpanel-background-color variable (--tabpanel-background-color: #181825 !important; for mocha), but there are still some issues with shadows (not sure if it can or should be fixed)

Without variable:

With the variable added: