Or you can do what @trevor did which was basically modify the Vanilla theme and learn everything from that repo itself.
Yup, Vanilla based and my theme is still nodebb-theme-vanilla. I just never changed it, but I plan on changing it soon enough because of the npm and nodebb upgrade conflicts of overriding everything.
Here's the process though... Interestingly enough I started by stripping out the things I'd prob never use in Bootstrap for a forum frontend (maybe backend but I stripped it out anyway).
And worked my way from the center in an outward pattern. So this means I began customizing everything from inside the <div id="content" class="container">...</div> then the header and footer. 8-14 hours a day, at 500~ iterations a day (because sometimes when you don't have a plan, and its just you, you ask yourself "does this look right?"). I use the same template pattern but add or remove different things.
I believe the LESS combined without including Bootstrap's Less and just my own custom one should be around 12,000 or so with about 2,000~ lines of code thats commented and commented out.
I've started on a theme I will release for the community but I can't give an ETA on this because I am hammered with other things at the moment. I am still learning everyday new things about NodeBB and how far I can push its limits which almost seem endless.