You can get a BA in whatever you want. Then you will get your MDiv. But, make sure you have enough undergrad credits of philosophy. I forget what it is now - something silly like 30, plus 12 credit hours of undergrad theology. It almost forces you to major in philosophy, or use all of your electives for philosophy classes.
But, if the diocese sends you to college, it might be hard to schedule that course load (graphic design) because they will have you at Mass every morning, praying the divine office, and a host of other things. From what I understand, minor seminary is focused around finished your studies so you can get to the real thing, but they still put you in the envionment to foster your vocation. (And the question may come up more than once - Why graphic design? For your parish webpage?

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Best thing to do is ask your vocation director. Then take a trip to the college they would send you to, go to the seminarian’s house and ask around. See what the schedule is like, etc.