@jongarrison you have another example on nodebb-plugin-custom-header. But in this case is managed by a plugin too. I don't know if it is possible to applu different theme (tpl). You could also managed it in the same tpl with a little bit of logic in it based in certain configuration server side.
Thanks for reporting the bug @mootzville, the templates property is no longer required in a custom theme's theme.json in v0.5.1 (but since we haven't even launched v0.5.0 yet, don't worry about a thing, the fix is backwards compatible)