The main layout elements are configured in the P2.Cuberry.Framework.Layout assembly. Styles, Brushes and some controls can be found there. Also the icons for the sub menu items are shipped with the Layout.dll.
The P2StatusStrip and the P2YellowDialog can be found in the P2.Cuberry.Framework.Helper assembly.
Some elements are kept in modules or in the main assembly, e.g. part of the MainMenuControl is defined in the P2.Cuberry project. Also the splash screen is in the P2.Cuberry assembly.
Standard icons can be found in the Layout assembly. Module specific icons are kept in the modules and can be found in the Images folder of each module. For additional styles each module has it's own Styles folder.
To change the icon of a sub menu item the icon name needs to be replaced in the column MenuSubMenu.ItemIconName.
New icons can be added to the Layout assembly and configured in the MenuSubMenu table.