Are you looking in the gem directory, e.g.,
$ cd `bundle show activeadmin`
Running find here shows (just a partial listing):
➜ activeadmin-0.6.0 find . -name "*.scss"
./app/assets/stylesheets/active_admin/_base.css.scss
./app/assets/stylesheets/active_admin/_forms.css.scss
./app/assets/stylesheets/active_admin/_header.css.scss
./app/assets/stylesheets/active_admin/_mixins.css.scss
./app/assets/stylesheets/active_admin/_typography.css.scss
... etc ...
./app/assets/stylesheets/active_admin/mixins/_typography.scss
./app/assets/stylesheets/active_admin/mixins/_utilities.scss
./app/assets/stylesheets/active_admin/mixins/_variables.css.scss
... etc ...
Those are the styles you can override.
Then however you're loading your CSS, for example:
➜ cat app/assets/stylesheets/active_admin.css.scss
@import 'active_admin_custom';
@import 'bootstrap_and_overrides';
Throw your overrides/customizations in there.