I believe that you need to update the styles in older documents. This is done through the Developer tab, which may not be showing on your Ribbon. If not, go to the backstage (Options)/Customize Ribbon, and check the Developer tab box. Press Okay.

Back in your document, in the Developer tab, select Document Template. If you check the Automatically update styles box in teh dialogue that appears, it will add the style --and add/change any other style changes you made to the template. If that's what you want, press OK. BUT! Immediately open that dialoge again and uncheck the box. Otherwise, styles will be updated anytime you make a local change in your document! You could, for instance, suddenly see all your text turn bold.

If you prefer to not update all your styles, only to add the one, the process is longer. Rather than writing it all out here, let me suggest a shortcut. Create a document including the style, with just a word or letter and save it. To copy the style to any other open document, have your cursor at the letter or word in this special doc and press Control+Shift+c, which copies the style and formatting, not the letter or word. Then, with your cursor in the document to which you wish to add the style, press Control+Shift+v. The style will appear in the styles list and you can add it to the Gallery.