so it is possible to make a (whole) user / character css layout toggle between dark and light, as an example? I've been looking for this, but I didn't know it was possible

Only the area after the toggle button inside the HTML (visually the button appears inside the box, but it's actually outside and before it and positioned inside using CSS) as this is utilising the CSS sibling selector which can only ever select things after itself—so you can toggle everything inside of the content area, but not the UI itself!

But yes, it would be possible to make a layout that can toggle between day and night mode.

You wouldn't be able to just plug in existing CSS, however, as it would require some code restructuring. If there's particular CSS you'd like to do this with, I'd contact the creators and ask them about it and perhaps they'll work with you on it (probably via a custom)!