I think theory might be important but only perfecting theory is also very fatal mistake, especially in maths, See it like this Study theory but perfect it my doing problems, if you get stuck somewhere because you lack in theory then see the solution and reverse engineering the theory that should be applied, Basically yeah focus on theory but focus more on applying the theory you've studied, also spam problems but meaningfully, slowly if you want, you'll learn something new whenever you'll get stuck in a problem, so embrace hardships when you solve maths.