"抽象主义理论" can be translated into English as "Abstractist Theory" or, more commonly and contextually appropriately, "The Theory of Abstractism" or "Abstract Theory" (though "Abstract Theory" is a bit broader and could refer to any theory related to abstraction; when specifically referring to the artistic or philosophical movement, "The Theory of Abstractism" is more precise).
In the context of art or philosophy, where "抽象主义" (chuàngxiāozhǔyì) typically refers to Abstractism as an artistic or philosophical movement, "The Theory of Abstractism" would be the most accurate translation. This phrase clearly indicates that it is the theory underlying or explaining the principles and characteristics of Abstractism.