Restyle letterbox to match Fx's updated In-content styling

The current iteration of vertical tabs in Fx Nightly features an In-content area that closely resembles Tor Browser's most recent version of letterboxing (aka "betterboxing"). The final design appears to still be in flux, however when these changes hit stable I think we should update our letterbox styling to match.

136_vertical_tabs_expanded

At present, I don't have any info on whether these changes will land before or after ESR 140. However based on the current designs in Fx Nightly those changes would include:

  • Restore the default browser background outside of the letterbox.
  • Apply the same border (if applicable).
  • Apply the same border radius.
  • Apply the same box shadow.

(Side note: I've noticed various experiments with the border, so I wonder if Moz have encountered the same issues we did in our implementation)

These changes should also close issues #42406 (closed), #42676 (closed), and address general complaints from users who found the gradient too distracting.

Edited by donuts