I used the notepad to change it completely. All menu files (except for pictures and so on) are located in UI folder (in pk3).
One of the things - you usually can find smth like this there:
rect 118 295 85 26
background "gfx/menus/menu_buttonback" forecolor 1 1 1 1
See " rect 118 295 85 26" ? That's the coordinates (118 295), and the size (85 26) of a picture used for certain element of the menu. We all know, that each picture in JA should be in power of 2, e.g. 1024x256 or 512x512, or 32x512 and so on. For this reason the two last numbers (like 85 and 26) are added, so that the picture looked better in the menu.
There're other things that U should keep in mind though (text, font, its size, color, etc.)