How to set global theme with arsd.minigui?

[TheZipCreator]

I'm using the arsd.minigui package for a project, and I want to 
add a dark theme/light theme switch. It seems that there's 
support for theming in minigui, but the documentation isn't 
exactly clear on how to do it. I found `DefaultTheme`, 
`DefaultLightTheme`, and `DefaultDarkTheme`, but how do I 
actually set my project to use them?

Also, the docs say:

> These don't apply to non-custom widgets! They will use the 
> operating system's native theme unless the documentation for 
> that specific class says otherwise.

But at least for me under Debian 12 (GNOME 43.9) it seems that 
the system theme isn't applying; the application appears in light 
mode despite the fact that I have my system theme set to dark 
(specifically `HighContrastInverse`).

I'd prefer users to be able to set their own theme, however, 
regardless of the system theme (if that's possible).