No, don't edit the sc_bootstrap.css file. It's hard to read and will be overwritten.
The SCLogin module has a built-in method to override our theme files. For more information on how, check out our
Joomla Login Themes
documentation.
Mainly, you want to copy the theme your using from the SCLogin parameters (either default or sourcecoast) from /media/sourcecoast/themes/sclogin/ to /templates/<YOUR_JOOMLA_TEMPLATE>/html/mod_sclogin/themes/
Then, edit away in that file. Your changes won't be overwritten, but you should make sure that you incorporate any changes in our theme files from the next update to make sure you don't miss out on some new styles.
I hope that helps explain, but if you have any questions, let me know!
Thanks,
Alex