Not a css guru so there migth be a better way, but if you create a class and set this on the reference selector that overrides the standard class this will change your icon.
I added the following class in the custom.css:
.myclass .glyphicon-share-alt:before {
content: "\2b";
}
Added the myclass to the reference set selector, this changes the glyphicon to \2b (plus sign).
Hope this helps you in changing the icon.
Might be useful to look into using fontawesome in your styling to achieve the same result.
Thank you @Erwin your workaround worked for me! Since it’s coded into the scss this solution is a bit hidden for the rest of the dev team. It would be great if this feature could be added to the modeller. I’m adding this to the "Idea” forum. Thanks again!
Idea forum link (go upvote!): https://forum.mendix.com/link/ideas/1283