If you are only making a few changes, then you can add them to the custom CSS feature with no worries.
However, anything that you add to that area (plus quite a few of the theme options, such as page width) gets added to the <head> tag and is loaded with each and every page. If your changes are significant, this can add to your server load, which is a consideration as you start to get more traffic.
If you use a child theme, that code goes into the child theme stylesheet, which is loaded once the first time that someone visits your page, and then is loaded from their browser cache as they move around your site. This reduces the load on your server.
Not only is the child theme for CSS files, it’s also good for the PHP files and custom functions. You just create a file with the same name and put in your custom function. This way the function will still be there after a graphene update which would override the original file.
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