CSS classes structure is created automatically, along with those you created:
portal portal-<portal-id> group-<group-id>
page layout-<layout-id>
[state-<state-id>] // exists, if the whole page supports states
place place-<place-id>
// state class exists, if the concrete widget supports states
[state-<state-id>] plug plug-<plug-id>
place place-<place-id>
// state class exists, if the concrete widget supports states
[state-<state-id>] plug plug-<plug-id>
place place-<place-id>
. . .
For example, this structure at user/list page in some state looks like this:
portal portal-user-list group-user
page layout-list
place place-a
state-loading plug plug-user-list-loading
place place-b
state-no-data plug plug-user-avatar-empty
place place-c
state-has-data plug plug-user-details