How can I change the normal App Icons

[AC Question]

I'm NOT searching for an icon pack!

A long time ago I installed a icon pack with automatically set the the icon of unsupported Apps to the same icon with a grey background (to fit the theme). However apparently when I deleted the icon pack the images somehow got transferred. Maybe it also conflicted with Nova Launcher I use.
Now all icons are set to this theme by default. In the settings, notifications, homescreen... everywhere the default theme with the grey background is set. Even for newly installed apps.
Somehow a transfer has happened where all the icons were transferred to the normal Android system. When this happened I had some apps with a special icon from another pack. This got transferred as well.
I want my normal "stock" icons back. It's so ugly in the notifications with the grey background.

My phone is a One Plus One running Cyanogen Mod based on Android 6.0.1, not rooted.

Is there any way to change these icons? Thank you for your help! If my description was to confusing feel free to ask please

 

[Tim1954]

If you're running Nova, in Nova settings to set icons to system or whatever...

 

[3P1C]

If you're running Nova, in Nova settings to set icons to system or whatever...

Jeah, I know. But exactly that's my problem. Somehow the system icons got changed to the old icon pack.
I can see this as well when I look into the system settings. At "Applications" all icons are also like the old icon pack even if I change the icons in Nova.

 

[Tim1954]

I'm wondering if it's a CM thing. My Nova is happy with running Stealth icons here on my S7 Nougat.. Our maybe the deleted icon pack sabotaged your icon settings on the way out..
What if you reinstalled that icon pack, set up how you want, then turf it again?