fix: changed source of night mode

[rodrigolanes]

Fixes #176 I'm not familiar with Anki's development, even so I studied the code and made the following adaptation:

old:

try:
    nightmode = mw.pm.night_mode()
except:
    nightmode = False

new:

from aqt.theme import theme_manager

try:
    nightmode = theme_manager.night_mode
except:
    nightmode = False

[ThoreBor]

Thanks! I'll have to check if this works on older versions as well, though