I have one more suggestion, which you may or may not use.
Well, there is a ready-made package that you can use to make the plugin use dark mode without worrying about such trifles as the lack of arrows next to drop-down lists.
It works as follows:
If your system has a dark theme -> Sigil also has a dark theme -> the plugin also has a dark theme.
It simplifies life because you don't have to play around with qss styles.
But you may have had another goal in mind, such as leaving the user to switch between themes.
Look at this screenshot and tell me, does your dark mode (in Windows) look like this too? Does it have arrows next to drop-down lists?
You will probably ask how to do that.
Just use a ready-made module from
here.