fix: sharp floating sidebar corners

[saadndm]

This updates userChrome.css to target #navigator-toolbox using the --zen-main-browser-background-toolbar variable, rather than forcing a background-color on hbox#titlebar, which previously caused sharp edges on the floating sidebar.

Fixes #56.

Original	Fixed

[neilyhy]

This now produces a border with default zen color
follows zen theme edit too
solved by adding --zen-main-browser-background-toolbar to :root

Zen version: 1.18.7b (Firefox 147.0.3) (aarch64)
on Macos

[saadndm]

Closing in favor of #61, their PR has this fix already and is more complete, @neilyhy please check if that issue is fixed in #61 and report there if it isnt, thanks.

[neilyhy]

Can confirm #61 fixed this