That’s the key there. Getting a solid handle on pure CSS will give you a HUGE advantage over just learning Bootstrap. Now, that doesn’t mean you can’t go on and learn BS after learning the core mechanics of CSS. BS basically (besides producing good looking sites out of the box) was made to speed up development by streamlining the process. You got your built in styles, you got your built in responsiveness and scripting functionality, it’s undeniably handy.
But you’d be seriously limiting yourself by only learning how to build with it. I give this scenario a lot but imagine you get hired by a firm that doesn’t use BS, or uses another framework entirely. You’d be stuck. Now let’s say you know both straight up CSS and BS, you’d be fine. In fact your CSS knowledge will probably help you learn any framework they might be using otherwise. Makes sense right?
My advice is make CSS your best friend and then branch out your development social circle with stuff like bootstrap, flexbox, and css grids.