How to show tags in posts with a theme that does not do it

Filter the_content, and add the result of get_the_tag_list():

add_filter( 'the_content', function( $content ) {
    if ( is_single() )
        $content .= get_the_tag_list( '<p>Tags: ', ', ', '</p>' );

    return $content;
});

The other option is to create a child theme and add the_tags() where you need it in a template.