Another question about color changes?!

Hey folks,

Ultra-beginner here. Trying to change the color of the background but to no avail. I would like it to be black like the immediate text background. Instead I keep having this white background clashing with the text column.

Here’s the page:
https://www.smontplaisir.com/presentation/

Any help would really be appreciated!!

The page I need help with: [log in to see the link]