Drupal core actually already provides classes that should be enough.
In template_preprocess_html() it calls theme_get_suggestions() and adds a bunch of css classes based on the path.
So for any path you are on that starts with admin there will be a class page-admin.
For example the path admin/content would have the classes:
and admin/structure/views would have
and so on.
It is best not to add more classes if you can get away with using ones that are already there.
And for the node add pages you can use the page-node-add class and for the node edit pages you can use the class page-node--edit.
That last one is a little confusing but it is because the path is node/%/edit and so the class is page-node-%-edit but then % is stripped out because it is invalid for a class.