That class expression is probably not correct. Can you share some examples of what the initialClass is for your thorium windows?
You can find this info by executing the following command in bash: hyprctl clients
If you want to rule out an issue with transparency, you can do something like this to manually set it and see:
hyprctl setprop address:0x[ADDRESS] alphainactive 0.5. Note that you need to supply the window address, which can also be found using the aforementioned hyprctl clients command.
For example, if the command prints this out...
Window 55fc96242940 -> Hyprland opacity rule not working for Thorium - SDF Chatter - qutebrowser:
mapped: 1
hidden: 0
at: 9,29
size: 1136,1402
workspace: 2 (2)
floating: 0
monitor: 0
class: org.qutebrowser.qutebrowser
title: Hyprland opacity rule not working for Thorium - SDF Chatter - qutebrowser
initialClass: org.qutebrowser.qutebrowser
initialTitle: about:blank - qutebrowser
pid: 1395291
xwayland: 0
pinned: 0
fullscreen: 0
fullscreenmode: 0
fakefullscreen: 0
grouped: 0
swallowing: 0
... then the command to manually set the transparency would look like this:
hyprctl setprop address:0x55fc96242940 alphainactive 0.5