Having chosen to use wedge combinations, Yoni and his team decided to focus the five themes strongly on the ally-colored pairs within each wedge (each wedge combination has only one ally pair), with the enemy color being a bit softer in the theme. This way, you could draft the ally pair to play the theme in two-color or add in the enemy color to play it in three-color. Here's how the themes played out:

Flying (blue-red-white)
The first theme they began with was flying. White-blue is traditionally one of the hardest themes to build around for beginners as it leans towards a reactive control strategy, which, while popular with experienced players, is very hard for beginners to wrap their head around. Flying, in contrast, is pretty straightforward. Play your evasive creatures and attack. Luckily, flying is primary in white and blue. This meant the third color with the theme would be red. Red doesn't normally have a ton of flying (usually just Dragons and Phoenixes), but enough that they could ratchet it up slightly without being too much of a color pie bend.

Elementals (green-blue-red)
Elementals as a creature type shows up in all five colors, but primarily is a red and green thing (red and green make up almost seventy percent of monocolored Elementals, as an example). Interestingly, blue is the third most common Elemental color (as blue is the color of air and water to red's earth and fire). This made it a good fit for the green-blue-red wedge. Elementals was also a nice theme because it tied directly to Chandra who was the main face of the set. (More on her below.)

Go Wide (white-black-green)
Green and white are the top two colors in both creature percentages and token making (green and white account for over fifty-five percent of all token making on monocolored cards), so it felt right to focus that color pair on a "go wide" strategy. "Go wide" means a creature-based strategy, often making use of tokens, to overwhelm the opponent by attacking with a huge number of creatures. "Go wide" strategies lean on making creatures and then effects that buff the whole team (also an ability focused in white and green). Black just so happens to be number three both in creature percentages and token making, which made it the perfect choice to be the third color.

Aggro (red-white-black)
This one was a little trickier. Most Magic sets like having an aggro strategy ("aggro" defined as using all your mana every turn to play creatures which constantly attack, trying to win the game as quickly as possible before the opponent can stabilize). White, red, and black are the three colors most associated with aggro strategies, but the two-color pair most connected with aggro is red-white. That's an enemy color, not an ally one. The idea for Core Set 2020 was to shift aggro from primarily red-white to primarily black-red with white serving as the third color. This required rethinking how some of the white and black creatures were positioned but was an attainable theme.

Control with "Enters-the-Battlefield" Effects (black-green-blue)
As I explained above, control strategies can be tricky for newer players. Yoni and his team decided that blue-black might be a better place to put the control deck as it's more based on doing things and less on reacting. Blue and black can both have creatures with "enters-the-battlefield" effects (ETBs), which can help control the battlefield in a more proactive way, something newer players can see even if they don't understand the card-advantage strategy behind it. They paired this with green as it has a larger creature focus, which plays well with ETBs. The one other reason this theme worked well was that the other four themes were all creature based, so using ETB-effect creatures allowed for more cross-theme synergy.