Are you looking to modify the look and feel for SharePoint. SharePoint lookbook is the option that is available to you.
https://lookbook.microsoft.com/
To understand how to design a SharePoint online site, refer https://spdesign.azurewebsites.net/
Now for editing CSS files, for SharePoint or non SharePoint, you may use Visual Studio Code,
https://code.visualstudio.com/docs/languages/css
https://code.visualstudio.com/
Hope this helps