I believe the current default theme is how GUROchan looked like through 2000s, or at least how GUROchan theme looks like on wakaba (because wayback archives don't have 2000s CSS saved on pages like https://web.archive.org/web/20080723182506/http://img.gurochan.net/g/wakaba.html )

It also seems to just have used kareha on some boards, sometimes with insufficient styling https://web.archive.org/web/20070601210628/http://orz.gurochan.net/req/index.html
Maybe we can use a similar layout on the text-heavy boards later on. That's what we used to do on /dis/ before switching back to vichan as I remember.

Later on it seems to have changed to this theme (which could maybe also be an option) with the board software changing to kusaba x: https://web.archive.org/web/20100611212411/http://gurochan.net/g/ (and seems to have stayed this way until its death, and at least the beginning of the second GUROchan iteration: https://web.archive.org/web/20140628020118/https://guroch.org/ )
It's mostly the same theme with a little lighter post background.

The Guro Light and Guro Dark themes (named Gurochan 2021 themes in the current selector) are a bit older than 2021 and due a renaming. They are from gurochan.cx, the iteration before 2021 reboot which died in 2020. This is a snapshot from September 2019 using the theme: https://archive.md/OH4Ss