1. Sports - One of the 4 main themes. Titles under this genre deal with sports of course, either Japanese modern martial arts like karate and judo or Western sports like soccer and baseball. Example: Captain Tsubasa and Slamdunk.
2. Judaimono - One of the 3 minor themes. Contents usually deals with historical facts of Japan or other places. Example: Kazure Okami.
3. Animals - One of the 4 main themes, and the most popular amongst all 7 themes. Manga under this genre will usually feature an animal(s) (or animal-like creatures) as the main character or as companions to the human characters. Example: Doraemon and Pokemon, Digimon included.
4. Cops, spies, thief - One of the 3 minor themes. Contents usually ranged from a detective with knacks of solving ridiculously hard mysteries to smart thiefs with golden hearts that likes to annoy the authorities. Manga under this genre can be insanely popular, but there are not very many of them who does compared to the 4 main themes. Example: Kindaichi Shonen No Jikembo and Detective Conan.
5. Shonen (adventure) - One of the 4 main themes, also very popular especially for teenage boys. Contents for this genre vary widely, but basically the plot revolves around single or a bunch of teenagers (with some guidance/help/participation from adults or senior citizens) battling against a single (or more) common enemy (enemies), with various ways like using mechanical robots, magical artifacts and superpowers. Example: DragonBall and Sailormoon.
6. Jobs - One of the 3 minor themes. Main character in the manga under this genre is usually a master in their profession. Example: Great Teacher Onizuka.
7. Shoujo (love comedies) - One of the 4 main themes, popular amongst teenage girls. Contents are rarely confined to this genre only, because from my observations, shoujo materials have the highest probability to be influenced by other genres, especially shonen and sport themes. Personally, this is my favorite genre outright. Example: Ah! My Goddess, Video Girl Ai and Ranma 1/2

Anyway, just because a manga/anime falls into any of the category above, it doesn't mean that that manga is not influenced by other themes. Most of the time, a good title is actually an emphasize of any of the themes above, supported by elements from at least one or more different themes. For example, Ranma 1/2 is basically a shoujo manga, but some martial arts and shonen elements were thrown in for good measure too. Another example is Kindaichi Shonen No Jikembo, a spy/cop manga touched with shoujo elements.