How can I make non-Upfront page content wider?

I created a page in the WordPress editor, not Upfront, and added content. The Default Template is selected in the WordPress editor.

I would like to make the page content wider. Right now it is rather narrow and does not match with the margins of my header and footer. How can I do that and have it applied globally for all existing pages that are non-Upfront created?

Sample page: http://musicologystore.com/music-lessons/make-an-appointment/ (public password = musicology)