How do I customize the appearance of sub-menus such as Items, Status, Equip etc?
The only customisation of the builtin 'special' menus currently possible is to change textbox styles and colours (in the User-Interface Colors and Box Styles editors) affecting the appearance of the boxes underneath the menus, and the text, in the Global Text Strings editor.
Customisation of the layout of these menus, using the slice system, is an in-progress feature which is already working but is currently hidden from users. You can either wait around (until early 2015) or ask for more information.
It is possible, with a great deal of effort, to reimplement most of the functionality of these menus with scripting (using slices) and then edit the in-game menu to open these scripted menus instead of the builtin ones.