Installation 

The extension has to be installed like any other TYPO3 CMS extension.

Composer mode 

Being a development tool, it usually belongs in require-dev — of the extension repository whose frontend is to be looked at, or of the test instance set up for it:

composer require --dev sbuerk/theme-extension-development
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A TYPO3 extension required that way is installed and activated exactly like any other one, because typo3/cms-composer-installers makes no distinction between require and require-dev. A deployment installing with composer install --no-dev simply leaves it out, which is the point.

Classic mode 

  1. Get it from the Extension Manager: Switch to the module Admin Tools > Extensions, switch to Get Extensions and search for the extension key theme_extension_development, then import the extension from the repository.
  2. Get it from typo3.org: You can always get the current version from TER by downloading the zip version. Upload the file afterwards in the Extension Manager.

Installing the extension does not render anything yet. The theme still has to be enabled for a site — see Configuration.