Great observation - but you need to ask yourself why did every page change!
You are working with a template which as is, designed, so everypage looks the same.
Look at top of the index.html page in the <head> section, and you will see
Quote:
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<link rel="stylesheet" href="style.css" type="text/css" media="screen" />
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This one line of code shows up on all your pages and tells the browser to look in that file for the instructions to set up each page.
You went in and changed some code, so all pages changed (to be expected).
You could go in an add code just below the code you want to change, give it a name and then call that part of code on the one html page you want to change. This opens up endless possibilities. You have to look at both the css and html files to compare them, inorder to see how things are done.