CSS custom properties (also known as CSS variables) are special entities containing specific values that can be reused everywhere in your stylesheet.

So, if you need to change a specific color accent in your document, you don’t need to run a global search to find out any occurrence of that color throughout the stylesheet. You just need to set a different property value within the :root block.

As an example of simple customization, say you want to set a custom background color. You don’t need to build a child theme for that. You just need to include two CSS declarations into the code editor of the Additional CSS panel: