I want to build my Gallery, but I don't like how it looks aesthetically. Is there anyway to use CSS like you can with your profile/pet profile(s)?

If not, I'll probably just turn my gallery into one of my pet's treasure...

Yes. Go into your gallery, choose "edit gallery", then you enter the CSS in the Text/Description: box.

Don't forget to go to your preferences and make sure you have Show Layouts in User Shops enabled, or you won't be able to see your edits.