wp-includes/class-wp-theme.php:550When converting the object to a string, the theme name is returned.
string public function __toString() { return (string) $this->display( 'Name' ); }Introduced in 3.4.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/class-wp-theme.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.