Video on course background color

Hi

Cant find any where how to change the color of the background in my course videos. Is it possible via css? How?

https://roadrunner-mauve-a2gn.squarespace.com/symbiotic-design-101/niche-khsz6-jnn28

to get in you must use a password and it is 12345678

it is a demo site i use for trying out things, before making them live.

Thanks