There was a moment I haven’t updated my TT-RSS instance and after upgrading I’ve lost my theme. It seems there were some change on this point.
I have found the good procedure on the Feedly theme github I use.
During my search I went on the wiki and remark the Theme link was not found on the homepage : https://git.tt-rss.org/git/tt-rss/Themes
I don’t know if you are aware of it ?
Have a nice day and thanks for your job !