Journal Style Query

[bedes]

Hello! I have a question regarding customizing my journal. More specifically, I'm making a side account, and I want to replicate the sort of look of this journal:

https://veryroundbirdfics.dreamwidth.org/

I realize that they're using custom CSS! My problem is I cannot, for the life of me, figure out what base code they're using. Is there a way to find what theme someone else is using, if they don't directly say it somewhere? If this isn't possible, but you think you know what base code is being used, shout it out in the comments!

Thank you!