A themed hackathon is one in which the projects are confined to a particular problem: such as food sustainability or returning citizens . Themed hackathons are able to attract subject matter experts (something that open-ended hackathons like Open Data Day DC are not good at), and projects typically revolve around problems that the subject matter experts bring to the table.
Should a logo be self-explanatory? It is only by association with a product, a service, a business, or a corporation that a logo takes on any real meaning. A logo derives its meaning and usefulness from the quality of that which it symbolizes. If a company is second rate, the logo will eventually be perceived as second rate. It is foolhardy to believe that a logo will do its job immediately, before an audience has been properly conditioned.
However, you do not refer to a type of product as a 'mark'. If you want to indicate which company makes a product, you use the noun make.