If You want to compose and thus express feelings, in other words, if the composition has to be unique and possess your own style, You can create your own theory. It is the matter of trying and experimenting on harmony, melody and other parts of musical piece. You can possess Your favorite chord progressions and cadences and simply use it! If You very like works of a certain composer, you should read the sheet music, listen to it and, maybe there will be an excellent chord progression or type of accompaniment. You can use it. But do not copy works of other composers, try to create your style and your composed pieces, which you will proud of.