can someone tell me why cb pro 8 needs aero theme?

i cant stretch the main window across 2 monitors

Cubase needs aero because of the Desktop Window Manager (DWM) funtionality, which uses the graphical processor (gpu) instead of main processor (cpu) for rendering the user interface.
Also the arrange-, mixer- and editor windows have there own task now. They can be stretched and moved on every monitor independently. The main window only contains the menu, which is not stretchable.

I just update from 5.5 to Cubase 8 and if Steinberg would agree to disabled this feature, it would be SO NICE !!

It needs aero enabled - what theme you use is irrelevant. With aero enabled the load is taken off your main CPU and system memory (if you have a direct x9 onwards card with discrete memory) as the GPU can provide direct memory access to the graphics “engine” in Windows - if you turn this off the video frames will be build on the main CPU in system memory and then transferred frame by frame to the video processing system - an enormous overhead. You should be able to stretch across multiple monitors - try removing the manufacturer’s drivers and running with the windows defaults.

also i see icons behind my project window

when u say video,u mean graphics not a video file?