Is there a repo/service/one-stop-shop for plain CSS framework agnostic elements?
The problem with using any of these is that you are very stuck in a certain design philosophy. Your choice also impacts what buttons, form elements etc look like. Also, don't get me started on things like jQuery dependencies. Mixing different elements from these frameworks really doesn’t work because your layout will be ruined. Why should wanting to use the bootstrap grid system also automatically decide what danger color a button has in your application? In the end, you spend a lot of time customizing these frameworks.
In my opinion what you really need is a grid system for easy responsive spacing, and then on top, you should be able to download elements in plain CSS code (or SASS, LESS, Stylus, whatever floats your boat) to not re-invent the wheel on every element. Also, there should be zero dependencies. It could be a community where people upload there programmed CSS elements and other people can download their code and upvote etc. Is there such a repo/UI search engine/product available that anyone knows of? Obviously, I know how google works, but haven't been able to locate what I have in mind.
There are minimal frameworks, containing e.g. just a grid. People often use one of these and then build their own components.
There are also component libraries. While they ideally aren't as opinionated as you describe (forcing you to use a specific color scheme), they still impose on you a certain philosophy and require you to use the whole library, not just individual components. Unlike e.g. Node.js where you can easily stitch together different things, UI requires consistency, both in terms of how it looks and how it's coded. Mixing different sets of components would not just be a nightmare in terms of appearance, but maintenance as well.
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All that said, you might like [Basscss](http://basscss.com). It's one of the least opinionated CSS frameworks, giving you just the "tools" part, not getting into appearance of your website at all.
So it would be something like