Need help by setting default-system icon to GroupBox

[vatriani]

At first ive try to explain why i need this and what ive expect.
Ive coded an collapseable QGroupBox, by using the checkable functionality. Its gonna work flawless. Here my code source-code at pastebin.

My problem is to get actual checkbox image overide by defining new stylesheet. (line 143 & 149) It works for own ressources-files like qt5-ressources.doc.
But i dont want to ship my own images, Ive want the native images from the os.
In the idea of qt5-stylesheet.doc So it says using ":/icons/..." will do a fallback to os-icons but i cant access them in these context.

So how can i change the images for these checkboxes without shipping my own graphics along? Thx for answers ;)