If you're Module Class Suffix is " my-module-sclogin", then the style you want to target is:
.moduletable.my-module-sclogin { // your styles }
You could also just target the style of:
.my-module-sclogin { // blah}
If you don't put the space before the suffix, then you can target .moduletablemy-module-sclogin The issue with that is that 'general' styles your template may have for moduletable won't work anymore.
The final caveat with that it's up to your template to properly include the module on the page in a way that the module class suffix ability. So, test with that setting, but it's not really a feature of the SCLogin module, it's a feature of Joomla that your template has to support for modules.
Further to my question - AND TO AVOID OVERWRITING WHEN UPDATING - so far, I've been messing around with the module's css and bootstap css files.
Generally, we recommend adding the styles to your own template file. Most template's have a 'custom.css' file or something similar to add your own styles. If you target .moduletables.my-module-sclogin .any-specific-style it should overwrite any of our styles.
I hope that helps, but should you need anything else, just let me know.
Thanks,
Alex