>My textbox base class (in my application) has a property .BACK_COLOR_FOCUS (current has a value hard-coded). This property would have to be set to the value oApp.BACK_COLOR_FOCUS (Access method of the oApp class). But I cannot set it in the visual class. In a PRG class that would work.

Here, this works (all in command window; curform.prg is on the path):

create class testThomas as textbox of libs\ctls
ox=curform()
ox.AddProperty("backcolor_x","=goapp.bkcolor")   && next I saved the class and closed class editor
goapp=createobject("empty")
addproperty(goapp, "bkcolor", 32)
_screen.NewObject("oxx", "testthomas", "libs\ctls.vcx")
?_screen.oxx.backcolor_x

Here's curform.prg, which you can actually do in command window line by line:

ASELOBJ(aa,1)
RETURN aa[1]

It doesn't really matter that there's a goapp.bkcolor_access method or not - I suggested having that method just for debugging, to see how many times it fires. The .bkcolor property (and other of your color properties) may as well be set in goApp.ReadConfig() method, or even hardcoded.