We humans love classifying things, compartmentalization helps us automate decision and make remembering easier. In code, we have something similar when it comes to styling the web you interact with. Imagine for a second that when you login into Hashnode all you see are words, no colors just black text on white background. I recon you won't enjoy Hashnode so much.
In terms of styling the web there are two major approaches; some developers use frameworks (an example of which is bootstrap) that allow them to issue grouped instructions using classes which have predefined characteristics. An example is like a basketball class, we technically expect people in that class to be of a certain height, wear the same uniform and exhibiting similar, coordinated actions on the court.
By using these frameworks we can issue blanket styles (like color) that will be reusable wherever we assign things to that class. The other way involves what is called "vanilla" styling where you decide with more specificity what each line of code looks like and does. While the frameworks are faster and less tedious it's also not as flexible as vanilla styling. Flexibility (when used well) makes code easier to read, maintain and extend.
It's easier to refactor code with vanilla styling than with framework styling as frameworks don't implement better code conventions that quickly. Vanilla code can however be a lot of trouble if it's not properly written. Frameworks offer standards such that you know what to expect, there's no good choice or bad choice, depending on what you're trying to do and your skill level, you can work with either framework or vanilla styling.