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.