When changing the design or css elements of your WordPress site it is better to make the changes in a child theme?
Because then the custom design changes you have made will not be effected (or lost) when you upgrade. To set up your child theme you need to create a folder in the wp-content/themes folder. In that folder you need to create a style.css file.
This file need to at least contain:
Theme Name: the name of your theme here
Description: your description here
Author: yourname here
Template: the name of the theme it is a child of here
Version: version numbering here
Once you have done this you should be able to select and install this new theme. The themes will use all of the CSS and other design elements of the parent (the original theme) except for anything you put in this theme. So if the default font color is black and you want it to be blue you would place that in the style.css file within the child theme folder. The same is going to be true with functions.php as well as other elements of some themes