Where you set the CSS will be dependent on what you have chosen for the "Theme" setting in SCLogin (used one of our default themes or created your own). If you're using our themes, they'd be in /media/sourcecoast/themes/sclogin/ directories.
If you're changing one of our themes to suit your needs, we'd suggest creating your own. You can read how to do so here
Let us know if you have any further questions.