There are two ways of adding body classes. The first is to add them in twig, the second is to provide additional classes in mytheme.theme.
Option 1: html.html.twig
Extend the current body class with the following:
{%
set body_classes = [
logged_in ? 'user-logged-in',
not root_path ? 'path-frontpage' : 'path-' ~ root_path|clean_class,
node_type ? 'page-node-type-' ~ node_type|clean_class,
db_offline ? 'db-offline',
current_path ? 'context' ~ current_path|clean_class,
]
%}
<body{{ attributes.addClass(body_classes) }}>
Option 2: define in mytheme.theme
In your mytheme.theme add:
function mytheme_preprocess_html(&$variables) {
$current_path = \Drupal::service('path.current')->getPath();
$variables['current_path'] = \Drupal::service('path_alias.manager')->getAliasByPath($current_path);
}
Then, html.html.twig:
{%
set body_classes = [
{{ current_path }}
]
%}
<body{{ attributes.addClass(body_classes) }}>