My icon theme is candy-icons and because of that I get these errors:
Dec 15 12:16:59 klauncher[937]: Icon theme "gnome" not found.
Dec 15 12:16:59 klauncher[937]: Icon theme "ubuntu-mono-dark" not found.
Dec 15 12:16:59 klauncher[937]: Icon theme "Mint-X" not found.
Dec 15 12:16:59 klauncher[937]: Icon theme "elementary" not found.
Dec 15 12:16:59 klauncher[937]: Icon theme "gnome" not found.

Dec 15 12:16:59 kglobalaccel5[959]: Icon theme "gnome" not found.
Dec 15 12:16:59 kglobalaccel5[959]: Icon theme "ubuntu-mono-dark" not found.
Dec 15 12:16:59 kglobalaccel5[959]: Icon theme "Mint-X" not found.
Dec 15 12:16:59 kglobalaccel5[959]: Icon theme "elementary" not found.
Dec 15 12:16:59 kglobalaccel5[959]: Icon theme "gnome" not found.

How can I stop the thing that looks for icon themes from looking for these icon themes?

Probably because you have something trying to use these icons. Are you sure you change the Icon theme globally?

FWIW: it is really helpful to specify which exact Plasma version and which distribution you use, it helps us answering...

I downloaded sugar icons theme from System Settings Add-On Installer and applied it from there.

Operating System: Arch Linux
KDE Plasma Version: 5.17.4
KDE Frameworks Version: 5.64.0
Qt Version: 5.13.2

Again, did you just use the icon them or did you change the global theme as well? That would be needed to prevent what you observe...

"candy-icons" theme comes with "Sweet" global theme. I'm using "Sweet" global theme right now but I'm stilling get those errors.

First of all: notifications about not found icon themes are not errors, but notifications. Where do you see those, btw, in the konsole output?

Do you use any non-KDE applications that have hard-coded icon requirements (anything Gnome, Firefox or else?)

I see them in konsole and journalctl is filled with them.

Yes I do use non-KDE apps too but the funny thing is that when I launched firefox through konsole, there was no "icon not found notification" but when I launched okular, there were those notifications.

Did you find a solution to your problem ?
There is a workaround found here : https://github.com/KittyKatt/screenFetc ... -323985143 , but would like to find a way to really fix this

So apparently the file "icon.theme" is checking for icon themes that were installed at some point, or are required by a specific application. I see no other solution than editing this file and remove the themes that are not installed as suggested by that article.

Launching graphical applications on a konsole will always print a lot of information, and again, those are not errors, those are notifications. If you don't want to see these, just start the application through the menu.