I'd recommend putting it in your theme actually. Don't edit this file directly as it won't stick around if you upgrade.
So instead of something like
.sourcecoast .btn-primary { /* styles here */}
It would be like
.sclogin .btn-primary { /* styles here */}
That should be the right style, off the top of my head with no testing
, but let me know if you run into issues.
-Melissa