I added some CSS options that were used outside Heyuri or created by users. To view them:

1- Go to [Settings] from top right (you will need JS for this, if you don't have JS enabled then you should know what you're doing anyways)
2- Click on "Style"
3- Select Futaba from bottom
4- Input each of these individually:

@import "https://static.heyuri.net/css/koko/usercreated/critea_space.css";
@import "https://static.heyuri.net/css/koko/usercreated/darkach.css";
@import "https://static.heyuri.net/css/koko/usercreated/fuuka.css";
@import "https://static.heyuri.net/css/koko/usercreated/gold_edition.css";
@import "https://static.heyuri.net/css/koko/usercreated/gorechan.css";
@import "https://static.heyuri.net/css/koko/usercreated/gurochan.css";
@import "https://static.heyuri.net/css/koko/usercreated/gurodark.css";
@import "https://static.heyuri.net/css/koko/usercreated/kurayami.css";
@import "https://static.heyuri.net/css/koko/usercreated/loliach.css";
@import "https://static.heyuri.net/css/koko/usercreated/loliboard.css";
@import "https://static.heyuri.net/css/koko/usercreated/midnight.css";
@import "https://static.heyuri.net/css/koko/usercreated/negachan.css";
@import "https://static.heyuri.net/css/koko/usercreated/sosach.css";

Credits:
kuz for sosach, darkach, fuuka, gorechan, gurochan, gurodark, loliach, loliboard, midnight
satancap for critea_space and gold_edition
anonymous heyuri user for negachan
komeo for kurayami
me for some tweaks