Hi,
The CSS file that gets loaded depends on which template pack is in use, e.g. Legacy vs Nouveau. Additionally, styles (CSS) can be customized via a Child theme and, if a Classic theme is in use, via the Appearance > Customize > Additional CSS menu path within the wp-admin area. Thus, which CSS file gets loaded is irrelevant/moot.
Specific to Avatars, How to Customize BuddyPress Avatars With CSS has some info (Note: might be a tad outdated). There are other sources on wordpress.org that contain general info regarding custom CSS, e.g. from the Advanced Administration Handbook – Custom CSS in WordPress
Hopefully, this is enough information to get you started.
