I'm familiar enough with common practice harmony, its rules and methods for diatonically harmonizing the scale, tonicizing scale degrees and modulating into new keys. But these tools seem to lose their relevance when applied to songs that use frequent modulation, modal mixture, non-functional progressions, parallel harmony, etc. Is there a theoretical system for making sense of music that falls outside the realm of common practice rules, which seem incredibly limited by comparison? Or is it open territory at this point, and "if it sounds good, it is good"? I'm talking mainly about jazz and blues, and pop music which uses the ideas in each.
I realize that's pretty vague– here are a couple examples:
Any help would be appreciated.