Sandra,
The file you'd want to edit to change the background of those buttons is /modules/mod_sclogin/css/sc_bootstrap.css However, you may just want to add the styles directly to your template. Specifically:
.sclogin .btn-primary {
background-color: #000 !important;
color: #fff !important;
}
.sclogin .btn {
background-color: #000 !important;
color: #fff !important;
}
If you add that to your template, it should change both buttons to white text on black button. Something to get you started that you can tweak from there.
Let me know if that helps, and if not, we'll gladly help further.
Thanks,
Alex