For that, you need to make up an external file (a text editor is fine for this), lets call it 'main.css'. It can have any name but the extension should be '.css'. Then put your declarations in there without the opening and closing style and comment tags, in other words remove:
Then on each page you want the styles to apply, put this in the head:
Make sure the href points to the actual name and location of your external file relative to each page this statement is on. The above statement is fine for an external file named 'main.css' in the same directory as the page(s) that are using it.
<link rel="stylesheet" href="main.css" type="text/css" />