CSS Architectures for UI developers

Writing good CSS (or whichever sub-flavour) requires a good level of experience.
In this moment of time, we are still trying to figure out the best way to scale CSS allowing maintainability and readability without sacrificing performance and ease of use.
Some good practices first, then methodologies and systems have been created. Learning, if not mastering them will allow you to approach any CSS framework or library, being it legacy or open source and create new ones with - still relative, ease.