The boots are important to me, too.
To me they look like Wellington rubber boots rather than platform boots, and on the initial walk up the driveway the shooter seems to have a bit of a "clomping" walk the way one has when walking in wellie boots, which by nature tend to be loose and bulky feeling.
Just about every woman in my social group, from ages 12 to 80, has a pair of Hunter brand wellies. They're mid-to upper-mid range in terms of price and accessibility. No one I know has white Hunters, though. Hunters are made in white but they're not the easiest color to find--more often you see traditional colors like black and green plus fashion shades like red. White is not the most practical color if you're going to wear the boots in rain or mud. White boots would show every speck of dirt. But, by their nature, wellie boots won't stain and can be rinsed easily.
So why would someone pick white boots in particular?
I tend to think that there is a cosplay link. Since the famous 1960s Courreges space era collection, white boots have been associated with a futuristic, sci-fi look. Someone might choose white boots for a specific costume or simply because they like that space-girl aesthetic.
Personally I don't think the shooter is dressed in a cosplay outfit for the attack. I think they are wearing a lab coat (easily stripped off and discarded) and rubber wellies (visible blood easily rinsed off) and I think this attire is consistent with the planning shown with the 2 am drive-by. But I think a cosplay connection might explain why they choose white boots in the first place, instead of more practical and readily available colors.