You can select it after you add a version.php file to it. Copy the Canvas theme's version.php and just change the plugin name 'canvas' to 'serenity'. But then there are places within the layout files that need updating as these changes will cause error messages to appear. So technically it's not going to work in every page in the modern Moodle sites like 3.0+ without a good overhaul which would be better to do perhaps in Base and Canvas themes too. It's a big job and I have lost heart in it all now, as things within Moodle 3.3 have changed so much and the new technology and ideas have taken the symplicity and joy out of creating Moodle Themes.