I can't find the right way to change the icon theme.
Here are the steps to reproduce the problem

1. New project > Graphical application
2. Add Button1 with Picture "icon:/small/floppy"
3. Try to set icon theme:

4) Yeat default theme from desktop environment is show on Button1 when application is run

(Click to enlarge)

PS: I am running Linux Mint 21.1 Cinnamon 5.6.8

 It has to be done early, like in the main form _init or likewise.
BTW, you only get one shot, if you miss then you're stuck with whatever is loaded by default.
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Thank you.