Bootstrap is far from being something you must learn, although is not going to hurt you if you do.
With the introduction of Flexbox I think a tool like Bootstrap is not as necessary as it used to be, but I as I said, learn well HTML and CSS, then if you ever need a CSS framework for whatever reason (say you are in a rush and have no time to style things yourself) then use a CSS framework like Bootstrap, you will only need to read the documentation, you won’t have to learn something new. I started my way from codding free slot games is was a hard mission.
Look at the design of this game – . I think you can find here more for your own. Bootstrap is a great set of tools/widgets for creating websites which target both desktop and mobile. Writing such multiplatform support code by yourself is cumbersome and error prone. Using it has another advantage: many programmers use Bootstrap to create websites. That makes it easier to work in a team.
Lots of places use it as a UI toolkit because it’s very easy to work with, but it isn’t essential (and it’s very easy to use if you know CSS/HTML). It’s just a set of CSS classes, basically. You structure your HTML like it describes in the docs, add the classes provided by Bootstrap. My first project was what do you think about it?