Change the color of the selected text

Dark theme and your installed plugins should not be preventing this change. Is Tools > Options > General > Opt-in to the editor beta enabled?

If so, disable it so the rule works. If you don't want to, wait for someone else to find the editor beta CSS selector for it. I'm waiting for this myself.

If not or no change occurs, check your CSS for errors preventing following rules from triggering.

Otherwise, try modifying the selection in development tools to see if it works as expected.