Hi Allison,
Thanks for submitting a question about menus and background images. Your theme has an “Above Header” menu area, which you can access from the Customizer. You can move your primary menu to that location in order to center it. I’m not sure what you mean in the second part of your question. Some of pages display background images and patterns differently. You can edit all of the affected sections in the Page editor.
Best,
Christopher M.