Light/Dark Mode Background Color Issue

First – I am seriously CSS challenged. I can made changes as long as I have Village Idiot instructions. Please don’t judge me. I’m in my 70s.

I chose Pliska for its Light/Dark mode user option and I love that. When trying to alter the background color I run into the following issue.

When I change the Light background to made the white a bit more muted, not as bright, it also changes the dark background to the same color and won’t let me change it back. I have to start all over on styling my dark mode and not change anything on the light mode.

Is this an issue in the dark mode CSS that can be fixed using the Custom CSS option?

What am I doing wrong?

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