This is an archive of all css from 4chan that I could find in the source code.
spooky.css - Spooky (Halloween Special)
yotsubanew.696.css - Yotsuba New
yotsubluenew.696.css - Yotsuba B New
futabanew.696.css - Futaba New
burichannew.696.css - Burichan New
photon.696.css - Photon
tomorrow.696.css - Tomorrow
yotsubamobile.696.css - Yotsuba Mobile
catalog_spooky.694.css - Catalog Spooky (Halloween Special)
spooky2017.672.css - Spooky 2017
catalog_mobile.694.css - Catalog Mobile
janichan.696.css - Janichan (/j/ special)