Websites use at least 2 languages to work, HTML and CSS.
CSS, which stands for Cascading Style Sheets, is the language used to format a site. Thanks to CSS, you can assign background colors, text colors, fonts, but also margins, positioning, in short, everything that has to do with appearance.
All WordPress themes have a
style.css file containing style rules to define the layout of the site's pages.
CSS is fairly simple to use and learn. There are many tutorials for beginners available on the internet.