Child themes are separate themes that rely on a parent theme for most of their functionality. If you are using a child theme, WordPress will check your child theme first to see if a specific functionality exists. If it doesn’t, it will use the parent theme. This is great because it allows you to modify only what you need.
Child themes should always be used if you plan on modifying even a single character in your theme.
If you plan to edit the theme files, like style.css, I`m suggesting you
optima-child.zipthemes, then activate the child theme. Child themes are great because allow you to edit the theme and also lets you update the original theme in the future, without losing your edited files work.