How can I change thickness of "outlines", which actually are not strokes but closed shapes? In the following example you can see outlines which I would like to make thicker, all of them. I wish I could convert them to strokes...
The easy way: add a stroke of the same colour, then
object > path > outline stroke, then merge using
Unite in the pathfinder window.
(or even easier: just add a stroke and leave it there so you can adjust it as needed!)
If you need to do the opposite and make it thinner by an even amount: add a stroke, outline stroke as above, then punch the stroke out of the original using
Minus front in the pathfinder window.