Update theme system loading to only be loaded once

[ssddanbrown]

Or at least work with certain artisan commands like config caching. For some reason that seems to be calling our functions.php loading twice, which throws error when functions are in the logical theme file, since it'll attempt to re-define them. Maybe check this is odd behaviour on the Laravel side, and not us. Otherwise, check if using require_once would have issues.

Related to #5121