gtk-demo/dnd: Fix, generalise detecting dark theme

Our default theme is now Default, not Adwaita, & HighContrastInverse was
renamed to Default-hc. So these checks did not work anymore. Rather than
hard-coding the new names, & possibly running into the same issue again,
we can just look for the convention of appending -dark to the theme name
and/or the Settings:prefer-dark-theme prop. The latter, we can & likely
SHOULD also apply to all themes - not just ours as before. We also check
for the :dark suffix as that means the theme variant - & before checking
GtkSettings check the GTK_THEME env var, just as GtkSettings itself does