When the Internet was young, styling attributes were stored directly within HTML structures. At some point, however, it was decided to separate these two components - structure and styling. You can still write them directly into the HTML code if you need to, but you usually resort to storing CSS in separate ".css" files and referencing them in the HTML header so that browsers load them and activate any CSS rules they contain.

CSS allows you to specify colors, fonts, margins, spacing and many other details. Nowadays, you can even make certain elements behave differently depending on the display device or available width/height - which is also called "Responsive Design": for example, wider monitors might display more elements side by side, while the same website, when loaded on a cell phone, might display the same elements below each other instead.