Hi Houston,
Sorry for the confusion.
That’s because Theme Options has higher priority than your child theme’s style.css
Adding the code in style.css will not always work as there could be other css code that override it but adding it in Theme Options will assure you that it has priority than all other.
If you have notice, my colleague have added !important
to the code which means that ‘the code is important, ignore subsequent rules, and any usual specificity issues, apply this rule!’
Hope that makes sense