The only page that I had to customize significantly was the homepage. Virtually all the content on the homepage is displayed via partial templates of other pages and liquid variables. This allows you to edit content via the standard NB content editors.
The content for the "I will volunteer..." panel is in subpage of the homepage called "Home_Volunteer." You can edit that text here.
The content for I WILL SIGN THE PETITION comes from the Sign the Petition page. The image of Brian Gagged is from the Petition Image page which is a subpage of Sign the Petition. If you want to change the image, just upload a new image to the files section of the Petition Image page. You can do that here. You don't need to add the image to the content as it pulls the file attachment itself. The rest of the text and video comes from the petition page itself. That content editor is here.
If you want to edit the content of the Donate page, you'll find that at Donate>Donation Settings>Intro.
If you want to change the video on the splash page, there is a page called Splash Video that has that content. You can change it here.