I was hoping to see if by "21st-Century" it meant "music theory that is applicable to the way modern music is generally written and interacted with". With details on things like microtonal music, increased focus on rhythmic theory, an increased appreciation for the role that timbre plays in the sound of music, the theoretical frameworks that underpin long standing musical traditions of other cultures and contrasting what makes each musical theory unique as a way of better understanding both western theories of music as well as the breadth of musical expression....

Nope! Just the same old, well beaten, rotting corpse of a horse in the form of a musical theory that entirely exists to prop up schenkerian analysis. The one that that racist, elitist music programs have been teaching since well before the 21st-Century. (The way you can tell is because it includes figured bass, which is totally irrelevant to any discussion that isn't about the harmonic style of 18th century european musicians.)

God forbid I want to, oh I don't know, make music that's similar to the kind that I actually listen to on a regular basis? Better understand its underpinnings and what makes it work in the ways that it does? No. Figured bass.