Hi,
Yes you can create custom.css file and you have to add the path on index. You can follow this documentation below:
https://www.joomshaper.com/documentation/helix-framework/helixultimate/custom-code
You can also add your CSS on template options,You can follow this path below:
Path towards custom CSS: Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.
Any custom CSS that you add inside Page is only available with that particular page. If you would like to use it on the whole website, then you will need to cut and paste it inside custom CSS area from your Joomla template.
-Regards.