Icon size in Okular and other applications

[solitone]

Hi,

I've got Plasma 5 on a HiDPI monitor (Apple MacBookPro 12,1, retina display). 
With some tuning, I've managed to set the right size for most icons on my 
desktop and many applications. 

However, there are some applications where icons are still too small. One 
example is Okular and the icons of the menu entries, as you can see in the 
attached screenshot. There are also other applications with similar little 
issues.

Is there anything I can do to change the size of these icons? Apparenlty, the 
settings on System Settings -> Appearance -> Icons -> Advanced do not affect 
the size of icons like these. What I notice though is that the All Icons entry 
is grayed out, and I cannot choose a size, but probably this is unrelated.

I'm on debian testing (stretch).

Thanks & Regards,
  Davide
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