Hi! i'm not sure which skin you mean specifically, but in any case, for my site skin i use a combination of multiple skins:
this is the base, with some colors changed: https://github.com/dawnsdarkest/AO3-SKIN-CSS-/blob/main/dark%20flat%20ao3%20skin%20v20.css;
this is for the flat buttons: https://github.com/electricalice/Ao3-flat-buttons/blob/main/flat-buttons.css;
this is to change the content icons on the works: https://archiveofourown.org/works/57331222/chapters/156317044;
and this is to change the stats icons on works: https://archiveofourown.org/works/55604875.

if it's about the socmed fic on ao3 i listed all the resources i used in the notes!
This for twitter (but in the css i made the property "background" and "color" in .twt "inherit," instead of a set color to make it work w users' own siteskins): https://archiveofourown.org/works/26754208

Reddit comes from this work https://archiveofourown.org/works/44783893 by mujiwrites, i derived the css and html by analyzing the page bc they haven't posted it anywhere yet, but if you want another option for reddit i used this previously: https://archiveofourown.org/works/33533569

I'll post the iOS chats code soon! it's based of this one: https://archiveofourown.org/works/77653461, and i can also suggest this skin for kakaotalk https://archiveofourown.org/works/54635311 and this skin for whatsapp https://archiveofourown.org/works/46531021