Can I modify Layouts or Sections?

You sure can! 

You’ll be able to tweak colors, copy, images, basically anything within the layout or section in a matter of minutes using Divi’s built in productivity tools.

Each section, row and module uses the Divi Builder’s settings as far as possible except where custom CSS is required which is included in a code module for you to edit. 

You can read more about how and why DiviWP uses Custom CSS here