I am updating a website created by someone else. It is easy enough to find the CSS rule that formats an element using Firebug's inspect feature.
When I change a rule, is there a tool that will tell me what other pages and elements will be affect by the rule change?
It is pretty easy to make changes that have unexpected side effects on other parts of the site. I am looking for a tool what will help me spot-check the site to make sure I didn't, for example, change a fonts color to gray-on-gray.
You should take a look at TopStyle Pro, it has a site feature, that spiders all the pages and tells you which ones use which CSS rules - should do what you want:
TopStyle's powerful Site Reports enable you to see at a glance where styles are used in your site. Find out where you've applied style classes that aren't defined in any style sheets, or see what style classes you've defined that aren't being used.