Aha.

Sometimes I'm dumb. Realized shortly after I wrote the last post that my parchment-colored graphic isn't a tile that's repeated; rather, each horizontal element is a single graphic, with the darker sides included. What a dumb decision the original template maker made! In other words, for the main "scroll" part of the blog (not the background off to the far sides), there are three graphic files the style sheet uses: A single instance of a top graphic which has the top border, a single instance of a bottom graphic which has the bottom border, and then as many instances of a middle graphics which contain bits of border on the left and right and which can be tiled only vertically to accommodate a vertically growing blog.

All of which is fine, except that is allows no expansion horizontally. Since the three graphics have fixed widths, I can expand the area where the text flows but I can't expand the "parchment" look that covers that area visually. I grabbed all three graphic files and could easily expand them in Photoshop to any width I wanted, but the template pulls them from a server to which I have download-only access - thus I can't upload newer, wider graphics to those locations. My only solution would be to host the new wider files somewhere else, and then change the CSS to point to the new locations rather than the old. Not very hard to do, and I may give it a shot soon. Until then, sorry about the annoying and unsightly right margin.