Provide modern theme for cZ

At the moment cZ is themed for the default theme rather than the modern theme. This is a first draft to make it look better.

[Approval Request Comment]
Regression caused by (bug #): n/a
User impact if declined: cZ doesn't look very good in modern theme
Testing completed (on m-c, etc.): 2.53.22pre
Risk to taking this patch (and alternatives if risky): low
String changes made by this patch: none

Comment on attachment 9496381 [details] [diff] [review]
1973583-irc-modern-skin-25322.patch

LGTM r/a+

The topic problem only occurs if the cZ window is so small with a height barely able to display a few lines of text. Unusable this way in any case. Further enhancements could set a minimum height.

Pushed by frgrahl@gmx.net:
https://hg.mozilla.org/comm-central/rev/40527a33c0ea
Provide modern theme for cZ. r=frg f=njsg DONTBUILD

Nice again in 20250823211717! :)

Topic div is scrolling away again at least since 2.53.23 beta 1 pre 20250908.

https://gitlab.com/seamonkey-project/seamonkey-2.53-comm/-/commit/140e76cf583d2cd684e84dffd6a97cb399b05c2d
Provide modern theme for cZ. r=frg f=njsg a=frg

I used to be able to copy the URL from the topic div to paste in an invite to a channel. That's not possible any more with its move to the status bar. Other than that, it's looking good again in 20251104212101 2.53.23 beta 1 pre.