First I have created an active_admin css file as app/assets/stylesheets/active_admin.css.scss
/* active_admin.css.scss */
@import \"active_admin
Based on seanlinsley comment on selected answer and nathan's answer too,
it seems to me that the 'cleanest' way is indeed to move active_admin.css.scss
from app/assets/stylesheets/
to vendor/assets/stylesheets/
.
No need to any other (configuration) modification: 'main' app has it's styles back to normal and AA display is normal too.
(I didn't want to manually include each of my ressources instead of require_tree ... !)
This is the way AA is getting into: AA (still) opened issue on the matter.