Brightside is a small utility for extending the functionality of Metacity, the default window manager for the GNOME desktop. Now at version 1.4.0, it currently offers two mutually exclusive functions: corner actions that are activated when the mouse cursor moves to one of the desktop's corners, and scrolling with the mouse between desktops.
I run the latest Ubuntu live-CDs inside a VirtualBox virtual machine to test them and run the graphical installer. Without this keyboard shortcut, using the installer would be nearly impossible because its window is larger than the 800×600 pixel resolution that VirtualBox uses.
When you customized your Ubuntu desktop to replace the brown theme, you may have overlooked the login window. GNOME’s login window, called GDM (GNOME Display Manager), can be easily customized by installing themes.