Atomic CSS

This explains the difference between inline styles and Atomic CSS: http://acss.io/frequently-asked-questions.html#how-is-atomic...

> How do you make this responsive?

Please find some explanation and examples here: http://acss.io/tutorials/responsive-web-design.html

Note that http://acss.io is responsive

> [How do you] improve on it after it has been built?

It depends on what you need to achieve. You can rely on "variables" to set values that can then be easily changed via the config (http://acss.io/guides/atomic-classes.html#variables), you can have classes that live in one single place (one single file to edit), or you can simply go "old school".

"Atomic" (Atomic CSS via Atomizer) can be mixed with any other kind of architecture as it has no footprint and it is not content opinionated.

CSS is all about trade-off - you gain here, you lose here. The architecture you choose depends on your requirements. Atomic CSS leads to very small style sheets and allows to style via markup. If you have no interest in those 2 "features" then it has not much value to you.