MingZ Do you know how to change the color to nominated color for the default avatar background color ?
I have tried files like :
1. /vendor/flarum/core/js/lib/helpers/avatar.js
2. /vendor/flarum/core/js/lib/utils/stringTOColor.js
3. /vendor/flarum/core/js/lib/models/Users.js
You should not change any of the core files, otherwise you will run into problems sooner or later.
Like mattsteele said use CSS to apply custom styles via Admin panel > Appearance > Custom CSS, e.g.
.Avatar {
background: black !important;
}
Otherwise you may write an extension to change the original behaviour.