Here what I had to do eventually …
I installed another fresh discourse docker … it worked prefectly … no issues … then I let me current discourse uses a fresh postgres database … it workled perfectly … so it seems some settings were messed up from the previous installation … the tables I suspected were: theme* and site_settings … I treid to fogure out which setting might cause this … no luck … or this will take a hell of time …
I was searching on how to reset these settings tot he default values and/or remove unused one …
eventually I neded up … for whatever theme I want to use, I just fork it on github, update it by adding the color definitions manually in one the scss files … and the install it from my admin interface …