After our informal poll here:
We now realize that nightmode is used much more often than daymode, so we've decided to make that the default color theme for new users and people who aren't logged in or don't have an account.
If you want to change it, logged in or not, simply press the black-and-white diagonal square in the top-right of the screen to switch from one to the other. If you are logged in, it will remember your preference. It looks like this: https://saidit.net/static/lightswitch-dark.png
If you have comments or questions, just reply below!