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Enable Theme Debugging on Development Sites
Theme debugging is a little more tricky because it's a service. Luckily, the example.settings.local.php file tells us how to enable local development services.
If we open up that example file, near the top, you'll see the "Enable local development services" section. As you can see it's looking for a file at "/sites/development.services.yml". That file already exists, so we can just copy this line into our settings.local.php file and add our configuration to the development.services.yml file.