Everyone opting for "do your own CSS" and not using a framework, is absolutely a horrible idea. You will not be the only one writing CSS for long term for your

Everyone opting for "do your own CSS" and not using a framework, is absolutely a horrible idea. You will not be the only one writing CSS for long term for your app I assume, any new member will find it extremely difficult where to modify following some good standards. They might miss a lot of things or overdo things perhaps. Regarding learning BS3 or BS4, I'd opt for BS4. All you have to know what things BS4 provide and use them appropriately. Not to mention, some fairly good CSS knowledge is also a pre-requisite. One of the themes we recently used is startUI (google for it). It's on BS4 and the components were easy to integrate in apps.