Using panel.css in K3?

how can I use a panel.css in Kirby 3?

I have added in the config

return [    
'panel.stylesheet' => 'assets/css/panel.css

and my css file in the asset-folder.
Also I do it like in K2 but it doesn’t work.


See Custom Panel CSS in the K3 Docs.

But Why I don’t find that if I type panel css in the search?
Btw. it shows me the panel options. but not that there is what I searched for. Thats a little bit exhausting. because that is not the first time I searched something and I got a ouput where I have to read all the input of every searchitem because I don’t get a preview if there is what I searched.