By default, the next puzzle for you is chosen randomly from within a pool of puzzles that are within a rating range close to yours. As you move up or down in rating your available pool of puzzles changes.

However, if you want to concentrate on a specific kind of puzzle, you can go into 'Custom' (unrated puzzles) and select what themes you want to focus on:

Then check the box of any theme you want to practice. You can choose one, or as many as you like at once!