[Theme: Theron Lite] Gallery background color

In the theme Theron Lite, I am trying to change the background color of the gallery to black, instead of the white seen below:

https://www.thecrowsneststudio.com/artists/vida/archive-of-the-works-of-vida/vida-1970s/

[Moderator Note: Please ensure that you are embedding links correctly in your posts. Link corrected.]

Additionally, I would like the text to be a pale grey, so as to be visible on the black background. We would probably be using hex #E6E6E6 or #F0F0F0.

Thank you so much for any help! If there is any other information I should be including, please let me know.