fix: get foreground color from GtkMenubar#menubar

[inukshuk]

Description of Change

Resolves #15194

This pulls the GTK menubar item's color from the theme's menubar item foreground. Previously, the menu item's color was used, which could cause issues with some themes (e.g., see the screenshot below using the Yaru theme, which is default in the latest version of Ubuntu):

cc @codebytere

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• PR title follows semantic commit guidelines

Release Notes

Notes: Fixes incorrect foreground color on GTK menubar

[codebytere]

Looks good to me as it stands, although ill wait for a few more linux people to give it some eyes :)

[ckerr]

LGTM. Thanks for the fix!

[Cobinja]

Tested with themes CobiBird, Adwaita and Adwaita Dark. Seems to work fine.

[release-clerk]

Release Notes Persisted

Fixes incorrect foreground color on GTK menubar

[codebytere]

@inukshuk can you open manual backports?

[trop]

I was unable to backport this PR to "4-0-x" cleanly;
you will need to perform this backport manually.

[trop]

I was unable to backport this PR to "3-1-x" cleanly;
you will need to perform this backport manually.

[trop]

I was unable to backport this PR to "3-0-x" cleanly;
you will need to perform this backport manually.

[inukshuk]

@codebytere I'd be happy to. Does that mean opening separate PRs in the 4-0-x, 3-1-x and 3-0-x branches? And backport the patch change to libcc I guess? Thanks for your help!

[Cobinja]

@codebytere Should this be backported to 2-0-x as well, or just starting with 3-0-x? Asking because you backported the menubar background color thingy to 2-0-x but this PR only got tagged beginning with 3-0-x