The “custom.css” file is the last CSS file read by browsers and, as such, essentially overrides other CSS rules for the core template. Further, your modifications within your custom.css file will not be overwritten if/when you decide you want to update/upgrade your template version/files.
My recommendation would be to inspect the elements you have been 9and continue to want to) modify, and paste the relative CSS rules within your custom.css file. It will save you much more time in the long run than it does to identify and paste the relative CSS rules within your custom.css file now.