Ralkage System time vs local time zone? May be a little complicated to do but not impossible right?
Both options should be relatively easy to achieve. The difference is doing it client side or server side.
The tricky part is generating two different stylesheets (and hopefully cache them) and serve the right one at the right time. Once you have two stylesheets it's just a matter of switching the style tag via PHP or JS.
And of course doing all that without Cloudflare trying to help.
