Aardvark

If you are using BuddyPress then this plugin controls the registration emails not the theme. To change BuddyPress emails see: https://codex.buddypress.org/emails/

If you're not using BuddyPress you can add the following filters to your child theme's functions.php file:

// Change admin's registration notice email subject
function ghostpool_registration_notice_subject( $blogname ) {
    return 'Hi [%s]';
}
add_filter( 'ghostpool_email_registration_notice_subject', 'ghostpool_registration_notice_subject' );
// Change admin's registration notice email text
function ghostpool_registration_notice_message( $message, $blogname, $user_login, $user_email ) {
    $message  = sprintf( esc_html__( 'New user registration on your blog %s:', 'aardvark-plugin' ), $blogname ) . "\r\n\r\n";
    $message .= sprintf( esc_html__( 'Username: %s', 'aardvark-plugin' ), $user_login ) . "\r\n\r\n";
    $message .= sprintf( esc_html__( 'Email: %s', 'aardvark-plugin' ), $user_email ) . "\r\n";
    return $message;    
}
add_filter( 'ghostpool_email_registration_notice_message', 'ghostpool_registration_notice_message', 10, 4 );
// Change user's email subject
function ghostpool_registered_user_subject( $blogname ) {
    return 'Hi [%s]';
}
add_filter( 'ghostpool_registered_user_subject', 'ghostpool_registered_user_subject' );
// Change user's email text
function ghostpool_registered_user_message( $message, $blogname, $user_login ) {
    $message  = esc_html__( 'Hi there,', 'aardvark-plugin' ) . "\r\n\r\n";
    $message .= sprintf( esc_html__( 'Welcome to %s.', 'aardvark-plugin' ), $blogname ) . "\r\n\r\n";
    $message .= sprintf( esc_html__( 'Username: %s', 'aardvark-plugin' ), $user_login ) . "\r\n";
    $message .= esc_html__( 'Password: [use the password you entered when signing up]', 'aardvark-plugin' ) . "\r\n\r\n";
    $message .= esc_html__( 'Please login at', 'aardvark' ) . ' ' . wp_login_url() . "\r\n\r\n";
    return $message;    
}
add_filter( 'ghostpool_registered_user_message', 'ghostpool_registered_user_message', 10, 3 );