Learning CSS will give you a better understanding of Bootstrap and other CSS frameworks. I think if you ever plan to modify the default Bootstrap stuff you are going to want to learn CSS. If you don't modify the Bootstrap stuff or just use themes from other people then perhaps you don't need to learn CSS but I think you would still benefit from CSS knowledge.
Besides Bootstrap there are other frameworks like Foundation that you can learn. Each framework may be different, but CSS will always be the same underneath them so if you learn pure CSS it's knowledge you can carry over to other things.