Too Long; Didn't Read
CSS is a fundamental pillar of the web, and like any fundamental thing, a deep understanding lays a foundation that will last a lifetime, assuming it will always stick around. Once you know CSS, you know it, and the best part is that no one can take that away from you. Understanding the underlying CSS will help you build with more ease with Bootstrap because you will understand the underlying principles that it is built with. Understanding how the frameworks that you use work and how they are built will help your workflow tremendously when you are actually using them.