A child theme is the safest way to modify an existing theme. But what if you just want to make a little CSS adjustment or add a few CSS lines?
In that case you can choose to use the Custom CSS box included in the themes instead. You’ll find it inside Theme Options » Design tab.
You can add there any CSS rules you want. Those rules will override the existing rules in the theme, and you can be sure they’ll stay there when you update the theme.
These lines will not get modified on “Reset”, but they will be replaced when importing options or restoring backups.