Check out this new button in the top right, it's a half-black half-white square.

Here's where it is, in the center of the green circle in this diagram:

https://i.imgur.com/TQkOW2m.png

If you click it, it will instantly change between dark and light mode. You don't have to be logged in. You don't even have to reload the page. You can just click it anytime and the colors change from day to night. Try it out! It's a new permanent feature of SaidIt.