the problem seem that use predefine id class name use by a css framework prevent you to use another framework !!!
I think there is no easy way to swicht from css framework to another one !
The id/class name are in the php code ( right ? ) so joomla must use dynamic id/class ? but the html tree can be different beetween css framework !
if you want to custom a template that use boostrap or another one that don't use boostrap ( flat UI, predefine tamplate not link to css framework ) ... the work is the same.
My review is that boostrap for designer is really vapor ware ( why learn photoshop dreamweaver illustrator ...during 3 or more years ) but for php and hikashop beginner it is a must have !!!
Joomla 3.0 ll use boostrap for backend, nothing force developper ( designer !? see after comment ) of extension to use boostrap !!
before boostrap , joomla have to use css/raster /javascrit in the back and Front end and it work's .....
Thank's zlenko a lot for UI kit information , it is the way i ll go but this need to study "All" the way to custom each HTML elemement since s same rendering can use many differents techniques (mix of css , raster, svg , javascript, ) .