Change CSS/formatting on specific page, but not pages in general?

How do I change the formatting on
http://wordpress.barrycarter.info/index.php/voronoi-temperature-map/
so that (for example) the map’s nearly flush with the bottom of the menubar?

I don’t want to edit page.php, since that’ll change all pages, not just this one.

After creating the specific CSS you need to format the page how you like, you could:

1. Create a custom page template with the CSS and assign the template this page.
2. Create a page-{slug}.php file with the CSS, where {slug} is the page slug for this page (most likely the filename should be page-voronoi-temperature-map.php).
3. Create a page-{ID}.php file, where if the page ID is 6, WordPress will look to use page-6.php.

These three options were mentioned in the Codex, under Template Hierarchy. By using any of these 3 options, you can alter the layout for this page without affecting all the rest of your pages.