There's no way to add the new badge to the old theme unless you add it to the news feed, if I do that then the badge will be resized small by the news feed parameters.
The old theme is just the basic bog standard board theme, it allows for very little visual altering.
It is extremely easy to just change your preference in your profile, the new theme is with the times and if the forum is to attract new members then it needs to cater for those new members, the fact you can choose in your profiles is a compromise.
NEW THEME OFFERS
A Clean and Uncluttered Forum face
Auto Embed of YOUTUBE and VIMEO links
Attractive colors in Manchester City colors as opposed to dull grey's and blue's.
Allows you to attach multiple images by uploading them to your post.
JUST INSTALLED BY ME FOR NEW THEME
You can now add your Facebook, Twitter, Google+, LinkedIn, YouTube, My Space, GitHub, VK, Spring, FQuick to add quick links in your profile.
Auto Embed of Twitter Tweets
Auto Embed of Facebook Posts
Embed Vine Video
Embed XBox Video
Embed ESPN Video
New Badge added to new Theme banner. As shown below