questions

Your GPA is fine. There would never be a minimum over 3.5, really over 3.0 should generally always be okay.

The shirts are a pretty solid white or blue usually, the majority of girls wear solid white/khaki/navy shorts, usually the shortest j crew are the most common for an idea. Honestly it's not important. But even in houses that wear Lilly, busting it out in round 1 is seen as kind of try hard by a lot of girls.

I agree with chill on the GPA, having a B in one class will not affect your chances everywhere and I say go ahead and take the class you need, school is the number one priority.
In regards to the shorts, wear what you are most comfortable in. If you wear Lily on a regular basis, then rock it. If you don't and would feel more comfortable in a pair of jcrew shorts that are khaki or navy, wear those. The girls will be able to tell more if you feel awkward in what you are wearing no matter what it is than if you are comfortable and confident no one will question your choice of shorts.