Create a custom template. You should be able to it from your current template, if you can figure out what files were modified. Simply copy those files to a folder you create. Folders should be on the same level as the existing classic folder.
For example, copy includes/templates/classic/css/stylesheet.css to includes/templates/YOUR_TEMPLATE/css/stylesheet.css.
Anywhere you see a /classic/ folder, you can create a folder for your custom template. You can copy the necessary files from either the /classic/ folder, the /template_default/ folder or the parent folder.
Edit includes/templates/YOUR_TEMPLATE/template_info.php to give your template identifying information so you'll recognize it in the admin under tools->template selection.