I've seen both sides of this idea many times, and from my personal experience, this idea looks good on paper, but usually implodes in practice. Bootstrap gives you a baseline quickly, but what pretty much universally happens is that at one point a feature requires you to venture outside of its constraints and that's when the trouble starts. Overrides and extensions, then something changes in the underlying theme or you update Bootstrap for some adjacent reason, that means more changes to the custom logic and eventually it's a big ball of yarn. Since you're working within the confines of Bootstrap and what it does and doesn't let you customize, you're never in a position where you can refactor all of it to make complete sense. In my opinion, if you're already working with a designer, it's way easier to build what they delivered from scratch, rather than try to bend Bootstrap over backwards to fit over it.