UI toolkits are useful for covering the edge-cases. No matter how much work you (or your team) put in, you simply can't test your own code to the extent that the Bootstrap community (or whoever) tests their code. Unless you have wildly esoteric UI requirements that you can't fulfil with an existing, open, well-supported framework it's always going to be more effective to put time in to learning and customising that, and getting all the benefit of tens of thousands of hours of people testing it on <i>everything</i>, than building something from the ground up yourself.