Can you change backgrounds globally?

Is there a way to change both the page properties background color and a grid's background color globally so it changes on every page in the site?

I sure hope so.

if all your pages are linked to a single CSS file then you can do it there.
If you are using CS2 then the "basic" css file which is automatically created when you open a new site "should" (I think) be linked already.

Depends on how you set things up when you built the site. If the background colors are managed in an external CSS file, then you just need to change the CSS file to affect all the pages. If the site uses templates and the backgrounds are set in the template, then updating that will fix all the pages linked to the template.