I have just used iDVD theme 7.0 Wedding, for a test project. If you import from iMovie with chapter markers, arrows will appear at the bottom of the screen to navigate left or right to the submenus but only if chapter markers are present. Navigation is achieved by pressing the left/right/up/down buttoned on a DVD player once you have written it to a DVD.
I have tested this using OS 10.8.5 ML and 10.13.5 HS.
I would seriously consider updating your OS to 10.8.5 ML in the first instance, then work on up to what your MacBook Pro will accept, check the MacBook Pro/OS compatibility chart, it is a good guide.
Be warned! As you update your OS you may lose functionality of some earlier favourite applications so check the applications/os comparability chart or check with others on the forums.
Note to all users of iDVD v 7.1.2 and iMovie 06 HD ( with a mod to the info.plist file). The best combination clearly is with OS 10.8.5 but both work perfectly with theme functionality in OS 10.13.5 High Sierra.
Just keep your Hard Drive clean and stay away from third party applications.