I originally used something else, but enough people complained about it being weird that I made the default a boring white/grey one. There are 10 other color schemes you can choose from in the dropdown in the footer though.

You can just send me an email to the address in the announcement blog post, and I can give you an invite: https://blog.tildes.net/announcing-tildes (that offer's open to anyone, but please read through the post so you know about the site's goals/principles/etc.)

It's not intended to be difficult to get an invite, I mostly just don't want to constantly deal with large influxes of users because of drama on other sites, persistent trolls, and things like that.

It doesn't show by default to logged-out users, but we've got a devoted group set up for Advent of Code that's reasonably active so far, if anyone's participating in that and would like to join us: https://tildes.net/~comp.advent_of_code