Magento 2 module to allow a file app/etc/theme-by-route.json to determine which theme needs to be loaded for which route.

composer require yireo/magento2-theme-by-route
bin/magento module:enable Yireo_ThemeByRoute

Create a file app/etc/theme-by-route.json similar to the following:

{
  "Loki/luma": [
    "cms/index/index"
  ]
}