[GameWizard]

You can do 1 of the 2 things, either make a plugin which should be relatively easy, but you'll need to have the appropriate hook to display the information in the correct place.

Otherwise, you can probably create a master style for all the styles you have, and then make the forumhome template adjustment in the master theme and it will then be applied to the rest assuming ofcourse you want the same ad to appear in all your themes, and assuming you have no unique forumhome modifications done in your styles.