This is how you add custom body css classes based on the "type" of the page (node or taxonomy page). Of course you can add whatever you want here. Even a css class for a specific taxonomy vocabulary or node bundle etc.
function mytheme_preprocess_html(&$variables) {
$route = \Drupal::routeMatch();
$route_parts = explode('.', $route->getRouteName());
if (in_array('taxonomy', $route_parts)) {
$variables['attributes']['class'][] = 'taxonomy-term--page';
}
if (in_array('node', $route_parts)) {
$variables['attributes']['class'][] = 'node--page';
}
}