While changing the theme background color to black through the customizer, you need to change the text color to white to make it readable. While doing that, the text in the comment form also becomes white making it unreadable because comment form is also white.
@manishsuwal I have confirmed this bug. Thank you! I'll get this fixed for the next update.
Fix bug with changing Comment Form Background color in Customizer
Props @manishsuwal and @diogeneskelsen. Fixes #173.