It should not be your css ... unserilaize is mostly used to unpack values stored in $modSettings The \ seemed like a path item which is why I said to double check the avatar thing.
Since you are in maintenance mode, can you edit the file sources/ext/serialize.php
throw new \Exception('safe_unserialize: unknown/malformed type: '.$type);
throw new \Exception('safe_unserialize: unknown/malformed type: '.$type.' string: '.$str);
so we can see the value that its failing on .. that may give us an idea of where in the ACP to look.