Here are various methodologies for structuring CSS:
OOCSS (Object Oriented CSS)
SMACSS (Scalable and Modular Architecture for CSS)
BEM (Block Element Modifier)
SUIT CSS (Style tools for UI components)
Atomic CSS
ITCSS (Inverted Triangle CSS)
Enduring CSS
Stylable is a pre-processor and component library that also has structuring constraints.
More will be added as discovered.
Here is a danger to be aware of.
Third Party CSS Is Not Safe