CSS (Cascading StyleSheets) allows you to format various design elements on your themes, such as spacing, colors, borders, indentations, animations, and a whole lot more.
Typically, in WordPress, editing a CSS file is done via the Theme Editor (Appearance > Theme Editor) on a file called Stylesheet (style.css) which controls the style and format of all the theme's design elements.
Additionally, adding custom CSS to your theme can be done via the WordPress Customizer (Appearance > Customize > Additional CSS)