In your Custom theme.css you can chose which font (Glyphicons is a font set) and the letter content code. You can do this by adding a class:
.glyphicon-minus:before {
content: "\2212"; }
The number in the content is the icon code. If you change the number in the content to the number of the font Icon you want.
Still works nicely. With chevron up and down:
.mx-groupbox .glyphicon-plus:before {
content: "\e114"; }
.mx-groupbox .glyphicon-minus:before {
content: "\e113"; }