The GUI implementation of gimp_wait() relies on the ability to run
plug-ins (namely, the busy-dialog plug-in) without entering the
main loop.  This prohibits the said plug-ins from making any PDB
calls, which would result in a deadlock.  However, we're currently
calling gimp_gimprc_query() to fetch the prefer-dark-theme option
during gimp_ui_init() (or any time the theme.css file changes).

Instead, communicate this preference through the theme.css file
itself, by writing a /* prefer-dark-theme */ comment to the file
when the option is set.  Yes, it's a bit of a hack :P