CSS variable `--color-primary-element-lighter` is removed

Ref https://github.com/nextcloud/server/pull/33946

it's used in 
https://github.com/nextcloud/spreed/blob/5bda7f0cecf939f52e8da199b28e21c830621446/src/components/MessagesList/MessagesGroup/Message/Message.vue#L986

should adapt to `light` I guess if it is not being restored.