Hi Jontox. I don't have experience with that type of monitor so I'm not really sure what the options are. As far as I know, AttractMode wants to run at the native Windows resolution. In theory you can force it in layout.nut by doing something like this:
fe.layout.width=320;
fe.layout.height=240;
fe.layout.preserve_aspect_ratio=true;
However I haven't had luck with that on my system, as it still seems to use a larger resolution than that. And I suspect it won't do what you want anyway since AM is still starting with that 640x480i resolution.
The other thing you can try is switching AM to windowed mode in the General options. That should allow you to size the window. Trying that I was able to get a 320x240 window, and this theme did look correct there. But in that case you'd just be using part of the screen so I don't think that is what you are looking for either.
Unfortunately I suspect that none of this will give you what you are looking for. Really it seems like you'd need to somehow configure the native Windows resolution at 320x240, but I'm not sure if that is even an option for your hardware.