I understand what you are looking for. Luckily you are using the Pro theme, so you have a complete control over your header and footer.
I suggest that you keep your website full-width as you'd mentioned with the 100% value. Then go to your header builder and add a maximum width for the bar or container as you have a separate option for them:
If you need more tips and tricks regarding the header builder I suggest that you check this and this article.
Now, the actual contents of the page in the builder is whole other ball game. You can add the background to the section, but if you want to limit the maximum width of the section you better use an inline CSS inside the row of that section. So you will need to go to the row options of that section:
max-width: 1000px for example, to the inline CSS option:
This will guarantee that you have a full-width section with the background, and the limited width contained row which the actual content of the section will reside:
I suggest that you check the
Cornerstone / Content Builder section of our Knowledgebase for more tips and tricks regarding the sections rows and columns, especially this article.