Update inactive tabs colors on tab tray when in private browsing

[topotropic]

Steps to reproduce

Visit a site in private mode and switch to non-private mode

Expected behaviour

Headlines and "inactive tabs" section have correct colors (what we currently have in non-private mode)

Actual behaviour

Headlines are white, inactive tabs section has accent color background

Device name

No response

Android version

Android 12

Firefox release type

Firefox Nightly

Firefox version

96

Device logs

No response

Additional information

No response

┆Issue is synchronized with this Jira Task

[rocketsroger]

This is dependent on #21897

[delia-pop]

Verified as fixed on latest Nightly 105 from 08/10. The inactive tabs no longer display the private mode color.

However, @MozillaNoah please note that other items on the tabs tray are still affected: the collection prompt (issue #22012 (comment)) and new tabs thumbnail images (issues #25977 (comment), #19226, #11421).

[MozillaNoah]

Thanks @delia-pop for posting your findings! I'll close this specific issue now then since there are other issues already to track those separate visual bugs. The others will be deferred until the rest of the tabs tray has been converted over to Compose, our new UI system, where it will just be one top-level change to resolve any more private theming issues.