Welcome

if someone wanna back to old steam theme chat, just leave steam and go in ''properties'' and put this: -nochatui -nofriendsui

in my case it is like: E:\Steam\Steam.exe -nochatui -nofriendsui (Cause i had moved steam recently to my SSD)

if you still found any problems, i did take a print/pic so check out https://imgur.com/a/M0hLqWi any doubt comment here down and i'll try my best to answer you

if you still cannot i think its because your Steam's property is set in ''Geral'' when it should be in ''Shortcut'' as my exemple is, i think creating a new shortcut steam in your workspace windows may help it, take care!

NOTE: it will only change the theme chat to old one, it won't effect neither change the other features like GUI/Menu etc

Don't forgot to leave or restart steam!