For a KS3.0+ 3D look you could also try setting the pens tag (KS2.0+ but it will work in KS1.3 fine because it's ignored), at the end of ExtNewScreen (right after gadgets and custom bitmap) and ExtNewWindow (right after screen type) structures, set a pointer to tags. Something like:
Code:
MyNewScreen:
...
DC.L 0,0 ; gadgets, custom bitmap
DC.L MyTags ; v36+
MyNewWindow:
...
DC.W CUSTOMSCREEN ; screen type
DC.L MyTags ; v36+
MyTags:
DC.L SA_Pens,MyPens
DC.L TAG_END
MyPens:
DC.W ~0
Also, WFLG_NEWLOOKMENUS is assembly symbol ($00200000), you OR (|) it with the other flags in the ExtNewWindow structure.