transmission-gtk does not follow any theming unless it's Yaru.

[thom]

This bug is present in Ubuntu-MATE 24.04 LTS Noble Numbat that has to come out in april 2024:
This bug is NOT present in earlier versions

OS: Ubuntu-MATE
Distributor ID: Ubuntu
Description: Ubuntu Noble Numbat (development branch)
Release: 24.04
Codename: noble
transmission-gtk:
  Geïnstalleerd: 4.0.2-1ubuntu3
  Kandidaat: 4.0.2-1ubuntu3
  Versietabel:
 *** 4.0.2-1ubuntu3 500
        500 http://nl.archive.ubuntu.com/ubuntu noble/main amd64 Packages
        100 /var/lib/dpkg/status

I did some testing and I have found that transmission-gtk has been compiled to use gtk-4
To get a dark background on a non-yaru theme (just for testing) i did:

cat <<\EOF >$HOME/.config/gtk-4.0/settings.ini
[Settings]
gtk-application-prefer-dark-theme = true
EOF

Which, indeed, worked.

So I think that most themes will need a gtk4.css or something like that.
Although it would probably better to compile transmission-gtk against gtk3, I am afraid that transmission will not be the only app compiled against GTK-4