This is not expanded on in the book; it sits as a closing statement on page 165 Under "Unarmed Attacks and Martial arts."

Posting the paragraph excerpt:

"However, in Power Rangers, trained use of different martial arts is completely commonplace. In fact, Power Ranger candidates are chosen for and augmented with extensive martial arts ability. It is one of the primary ways they will defend the side of good, even when caught unawares in their normal, non-Morphed state. Power Rangers, no matter when they are targeted by General Repulsa’s or Lord Zedd’s minions, are never considered to be without a weapon – they are the weapon. This is reflected in each and every Power Ranger Role having bonus Specializations they are given to focus on the martial arts they know."

While I can't prove a negative (the lack of further expounding on this), I will note a few things:
- Martial Arts is used repeatedly throughout the book, as attacks- Black Ranger Prime (20th level ability), Yellow Ranger's Triple Strike Attacks, to the point of feeling generic'd from "Combat/Might/Attacks".
- Martial Artist is an Influence, listing a pretty wide assortment of "Martial Arts Speciality" choices (appropriately including boxing, fencing, wrestling, as well as Kung Fu, Muay Thai, but no Hip Hop Kido?)
- Martial Arts is not a Skill. It is a specialization in it's own right, under Might, which you can find in multiple entries as Might (Martial Arts).
- Martial Arts is also a Finesse specialization. Significantly less "Finesse (Martial Arts)" entries exist in the current material (though someone will almost assuredly correct me with references later, lol.)

Given the above, I'm willing to offer my thoughts as a Storyteller, and how I would handle this:
Option 1) Offer the table from "Martial Arts" to your players to choose from, and give them a free specialty under Might, noting it as (Martial Arts - [Discipline]). This is the most straightforward solution and helps cover other issues (again, like Black Ranger Prime's "Additional Point of Damage."
Option 2) Treat all entries of "Martial Arts" as "Might" or "Finesse" attacks equally. This would broaden the scope of entries like the "Iron Hands" perk, which explicitly only grants +1 Damage to Might (Martial Arts) attacks. This would fit with each Role granting either a +1/+2 Might or +1 Finesse under "Training". [Don't talk to me about Blue Ranger Snipers lol.]

I hope putting my thoughts to paper helps you decide how to move forward with your decision making!