Created attachment 151290 [details]
windows and gtk3 backend
Menubar -> Tools -> customization
at gtk3 and qt5 backend the arrow icons were shown and on the windows backend there were some self rendered images were shown.
please use also on windows the icons from the icon theme.
Confirmed on Windows 10 Ent 64-bit en-US with recent master/6.3.0 build
Build ID: 85c0521f01f5c726e9f754b3175a550121e566c8
CPU threads: 8; OS: Windows 10.0; UI render: GL; VCL: win;
Locale: en-US (en_US); UI-Language: en-US
Looks like the stock UI icons used on Windows are old Industrial/Galaxy, maybe? Or something earlier...
But IIUC don't think the button's assigned icons (image1 - gtk-go-up, image2 - gtk-go-down, image3 - gtk-go-forward, image4 - gtk-go-back) come from UI , rather they're linked from source. Where they are mapped to stock SymbolType . And I've no idea how the icon theme for os/DE gets picked up for use there--if it can.