Fruit Ninja doesn't really need a brief. It's a classic mobile game that has been around since I could remember. It's had some updates though so I thought I would take a look at their new look and see how they're faring in the cutthroat mobile market.
The theme of Fruit Ninja UI is meant to be based off the archetypal Ninja with touches of ancient Japanese flavor in the scrolls and parchment. It's kept light, however, with it's cartoon style characters and bright colors. When looking at the layout, there's no clear theme as each screen was probably designed independently of each other. Some things are kept consistent such as the bottoms of the "scrolls modal" always has some sort of action to it. Whether that's the X to close it or a button to change blades. There can be awkward situations, however, when screens get a modal on top of it which get another modal stating an error and it just looks a bit messy.
There are 2 main typefaces in Fruit Ninja. The title font is the unique Fruit Ninja font that was probably made specifically for Fruit Ninja. The 2nd font is a playful sans-serif font with a bit of flourishes in the T's and L's. This font is also fairly round to give it a fun and friendly look that is well suited for kids. The Fruit Ninja font is notable because of it's Kanji style letters. It reads as English but looks like Japanese (roughly).
Fruit Ninja uses a parchment brown for most of it's screens with various colors throughout. The various colors are all over the place depending on the thing it's representing. For instance, if a number is associated with how much Starfruit you obtained, the number is green. Also to note, much of the buttons in the game are yellow with the few exceptions being any button that costs Starfruit to press is green (like the number). This color differentiation is important to reinforce the face that Starfruit is their hard currency and whenever you pay with Starfruit, you might be paying the equivalent in really money.
Fruit Ninja is a good example of using the mechanics of the game in the UI. For important decisions, like playing a game or exiting a game, you have to slice a fruit. This acts as the button for that action. I really like this way of teaching and reinforcing mechanics through UI. However, I thought their tutorial was a bit overkill. With such a simple game I don't think you need to teach much. Let the player explore the game on their own and just slice fruit.