Or Star theory. Or star theory. But never star Theory. The capitalization is disputed, but that doesn't matter.
If you paid attention while reading Dark Thresholds, assuming you read it at all, you've seen it mentioned. But what exactly is it, you wonder? Well, the answer is quite simple.
What Twilight told you about the theory is true, but it's incredibly far from complete. This hypothesis is a minor obsession of mine, and it's been investigated beyond what is healthy and extended beyond what is reasonable.
For those who don't know, Star theory is the idea that star shapes in MLP, especially cutie marks, represent magic, and furthermore that the number of points on the stars represents relative magical power.
Long story short, I have a belief that Star theory might be canon.
Let's go on to the evidence. I'll organize it in order of pointation, starting with four.
Note: The three-pointed star is not canon, it is merely an addition in Dark Thresholds fanon to describe below-average unicorns, such as Snips and Snails. Most can do nothing more than illumination and telekinesis. Technically, Stella would be classified as three-pointed.
Four Points: Four-pointed unicorns can do simple spells (illumination and telekinesis), as well as one or two other spells specific to their special talent, unless their talent is magic. Rarity is an example of a four-pointed unicorn (the gems in her mark have four sides each, and she's often surrounded by four-pointed effects). Her talent spells are gem-finding and magicking clothes onto things, such as the vest onto Twilight in Winter Wrap-up. Blueblood is a four-pointer whose special talent is magic, as are the Royal Magi that dance with Rarity in 'At the Gala.'
Five Points: Five-pointed unicorns are above-average, and their talent is typically magic, but there are several exceptions. These individuals are rare, with most examples being background ponies (the yellow one that tries Flim and Flam's cider and says "I wouldn't pay one cent for this dreck!" for instance), but if you look around you can find some. Trixie's getup is covered with five-pointed stars, and the minor star in her mark is five-pointed, suggesting that that is her pointation (though her talent is likely showmareship and sleight of hoof, not magic itself). Twilight's Star Swirl the Bearded costume is also covered with five-pointed stars, and above the Star Swirl wing in the Canterlot Archives there is a similar star. Add to this the fact that the previous-gen Star Swirl ponies all had five-pointed marks, and we can assume the same for him. Twilight's mother has three pentagrams as her mark. Also, all Royal Guards have five-pointed stars on their breastplates, and these stars can be found all over the place in general, especially Canterlot.
Six Points: Incredibly rare. These are very very powerful sorcerers. That's all I'll say before I risk spoilers. Twilight is the most obvious example, with this star adorning her flank, as well as most Twilight Sparkle paraphernalia (including the sparkle in her eye). The Element of Magic (the stone version shown momentarily before Nightmare's banishment) has it as well. Shining Armor has one on his mark, and the thingy he wears to the wedding bears it (he claims that it was his uncle's, implying that the star runs in the family). Only one background unicorn has a hexagram as his mark--on mlp.wikia.com he's listed as Ponet, but to me, he's Parallax.
Note: The trio of stars on Allie Way's flank aren't necessarily covered in the theory. First of all, most of the time they must be centered on the mark. Secondly, those are each probably just the intersection of three lines, anyway, not really stars.
Seven Points: Doesn't exist in canon--only exception is a star atop a tree in Hearth Warming's Eve, which we have yet to fully explain. In Dark Thresholds continuity, this is the level of Aurora Spark.
Eight Points: Celestia's sun mark, as well as most sun symbols and starbursts associated with the Royal Sisters, has eight points. The stars on the Ursi's heads have eight points, implying that at least the Major is as powerful as Celestia. Some clocks have eight ticks, but this probably doesn't mean anything.
Two Points?: Luna's mark, as well as Twilight's fathers', have crescents, which some (of note, SSky) interpret as a two-pointed star. Personally, I don't prescribe to this theory (we describe crescents differently), but SSky believes it represents void magic.
Additional Evidence: Look around! There are stars everywhere--they're all over the roofs of Canterlot! Also, Celestia's chariot as it appears in Swarm of the Century has stars going down its edges, in order of pointation, with six-pointed being the highest save for a single eight-pointed star on the headboard.
So, what does this mean to you? Well, you may use this theory as much as you like as long as you credit it (at the very least, when asked) to me and/or Asgardian. So far, the only one who's used it is SSky, in "With Wings as Eagles" (the spires on top of the thirteen towers, among other things), but I sincerely hope it will catch on.
Also, if you could get in touch with one of the show's creators and perhaps ask about this, I would be grateful. I've tried in the past but gotten no results.