Thanks so much FaLLeNOnE. Your suggestion of moving AFA_OS_LOADER just before IPrefs worked. My backdrop was still dithered, but I chose a 16 bit screenmode instead of 15 bit and this is now fine. Strange because 16 bit screenmodes never worked before!
I now have just minor problems with the appearance of the text. As you can see
here, the background colour of inactive window border text is grey instead of transparent. In the active window it looks fine. Also the text on the Workbench title bar is too far over to the left so the boing ball overlaps it. The text used to be to the right of the boing ball before I installed AfA-OS. Finally, the window border text is not vertically aligned, whereas it was before.
Any advice on these minor problems would be really appreciated. Thanks again for your help so far.
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moto