How to set global theme with arsd.minigui?

[TheZipCreator via Digitalmars-d-learn]

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).