You have three Gallery Layout Design options that can be modified to change the design of your Collections:
Grid Style: Vertical or Horizontal display for your photos
Color Theme: Light or Dark background for the Gallery
Thumbnail Size: Regular or Large display size for your photos
You can find these options by going to the Collection > Collection Settings > Design > Gallery Style.
A Vertical grid emphasizes portrait photos, and a Horizontal grid emphasizes landscape photos.
Choose between a light or dark theme that best highlights your photos.
Adjust the display size of photos in the Gallery.
Tip: Want to select the same specific design options for all of your Collections moving forward? You can easily set this up using Collection Default Settings!