Go to settings -
Then go to Style Settings> Avatars> Default Text for Avatar
And we prescribe the unicode of the desired icon there, you can see here - Font Awesome Cheatsheet.
fontawesome.com
Put extra.less:
Result:
/admin.php?options/groups/users/
User Preferences and disable - Enable dynamic avatars by default.Then go to Style Settings> Avatars> Default Text for Avatar
And we prescribe the unicode of the desired icon there, you can see here - Font Awesome Cheatsheet.

Get a Version 5 Cheatsheet for Unicode & Glyphs | Font Awesome
Need a Font Awesome icon's glyph, name, or unicode value? Use this Version 5 cheatsheet for quick copying and pasting.
Put extra.less:
Less:
.avatar {
&.avatar--default {
&.avatar--default--text {
> span:before {
.m-faBase();
}
}
}
}