The subject here may be one or more of the elements or principles of design. For example, it might be a painting about color or repetition or movement. Paintings like this can stand on their own or work as a plan for a general idea with some reference to a subject. What about style? Style is a natural development, not something you arbitrarily decide to do. It happens naturally. Your style and subject fit together perfectly if you're painting the right subject for you. Your interest in the subject shows in your work.