When I hit the power button of my computer, a window titled 'Shut Down the Computer' appears. It gives a few options, including shutting down or restarting.

This window is different following the context I hit the power button :

• when logged in and seeing my desktop, it includes colourful icons (red for shutting down, for example).
• when I am not logged in yet, the icons are monochrome and a little bit more stylized.

I clearly prefer the last type of icons : how do I set them as default whatever the context of use of the 'Shut Down the Computer' window ?