To change them all, add this to your stylesheet:
To target specific groupboxes, use a class and put this class in front of the statements in the snippet, with a space.
You can do this by using CSS. Give the groupbox a class for example: groupbox_example
and then use the css below:
.groupbox_example i.glyphicon.mx-groupbox-collapse-icon.glyphicon-minus:before {
content: "\e900";
font-family: 'icomoon'; }
.groupbox_example i.glyphicon.mx-groupbox-collapse-icon.glyphicon-plus:before {
content: "\e902";
font-family: 'icomoon'; }