Student or Learner
I know the general meaning of the word "theme". But in the following context, it's confusing.
So he decided to allow people to have the functionality of Timer for free, but to let people spend more on it if they want to with in-app purchases (IAPs). The IAPs were simply different themes: aesthetic choices which allowed people to customize how the app looked and the sounds it played when the timer was up. They started at 99 cents. The most expensive IAP was the Ultimate bundle, $9.99 for all the themes and sounds both in existence now and that App Cubby might release in the future. None of the IAPs were necessary to use the app.
not a teacher
A theme can be an idea or look that is carried through all aspects of something. A restaurant might have a unifying theme of Parisian style or classic horror movies etc, or an amusement park might be based on a theme like the Wild West or space travel. For things like apps and website design, a theme is often a choice of colours, typography, sounds etc ("aesthetic choices") that the user can manipulate to give the app or site a unified appearance and flavour that they particularly like. So the common factor in this meaning of "theme" is the sense of a distinctive idea or appearance that is carried through the project.