I'm attempting to override Atto plugin icons in my Boost-based theme in Moodle v3.4.
Seems like there isn't a consistent or easy way to do this. There's this discussion in the Text Editors forum describes the problem: https://moodle.org/mod/forum/discuss.php?d=355146
I'm just linking that discussion here in the hope that maybe a themer could shed some light on that best way to do this.