small tweaks?

[littlewhitecloud]

LOL Maybe someone like me often write set_theme("Dark"), so I think we can add .lower() to avoid the check fails.
Also think we can use

cls.set_theme("dark" if cls.get_theme() == "light" else "light")

instead of

if cls.get_theme() == "light":
    cls.set_theme("dark")
else:
    cls.set_theme("light")

[rdbende]

Thanks!