If you really are on J3.7.0 update immediately to J3.7.1 as it has a serious security flaw.
Use this code...
[class^="icon-"], [class*=" icon-"] {
width: auto;
}
To create a custom CSS compatible with Gantry 5 please read this
http://docs.gantry.org/gantry5/tutorials/adding-a-custom-style-sheet
.
Remember to recompile CSS from base outline too.
Regards, Mark.