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( $gp_blogname ) {
    return 'Hi [%s]';
}
add_filter( 'gp_registration_notice_subject', 'ghostpool_registration_notice_subject', 10, 5 );
// Change admin's registration notice email text
function ghostpool_registration_notice_message( $gp_message, $gp_blogname, $gp_user_login, $gp_user_email ) {
    $gp_message  = sprintf( esc_html__( 'New user registration on your blog %s:', 'gauge' ), $gp_blogname ) . "\r\n\r\n";
    $gp_message .= sprintf( esc_html__( 'Username: %s', 'gauge' ), $gp_user_login ) . "\r\n\r\n";
    $gp_message .= sprintf( esc_html__( 'Email: %s', 'gauge' ), $gp_user_email ) . "\r\n";
    return $gp_message;    
}
add_filter( 'gp_registration_notice_message', 'ghostpool_registration_notice_message', 10, 5 );
// Change user's email subject
function ghostpool_registered_user_subject( $gp_blogname ) {
    return 'Hi [%s]';
}
add_filter( 'gp_registered_user_subject', 'ghostpool_registered_user_subject', 10, 5 );
// Change user's email text
function ghostpool_registered_user_message( $gp_message, $gp_blogname, $gp_user_login, $gp_user_email ) {
    $gp_message  = esc_html__( 'Hi there,', 'gauge' ) . "\r\n\r\n";
    $gp_message .= sprintf( esc_html__( 'Welcome to %s.', 'gauge' ), $gp_blogname ) . "\r\n\r\n";
    $gp_message .= sprintf( esc_html__( 'Username: %s', 'gauge' ), $gp_user_login ) . "\r\n";
    $gp_message .= esc_html__( 'Password: [use the password you entered when signing up]', 'gauge' ) . "\r\n\r\n";
    $gp_message .= 'Please login at ' . home_url( '/#login' ) . "\r\n\r\n";    
    return $gp_message;    
}
add_filter( 'gp_registered_user_message', 'ghostpool_registered_user_message', 10, 5 );