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:
Public Sub Form_BeforeArrange()
Application.Theme = "Gambas"
' Application.Theme = "Gambas-Mono"
' Application.Theme = "Adwaita"
' Application.Theme = "Papirus"
End
4) Yeat default theme from desktop environment is show on Button1 when application is run
PS: I am running Linux Mint 21.1 Cinnamon 5.6.8