I reveal the secret for everyone is interested

h34r:
If you've taken
Default Firmware 3.0 Skin with install script , it's not good as there aren't "credits.ini", "skin.ini", "system.ini" and it isn't enough to replace button.png of "imgmain" and "imgsetting".
Instead, if it uses a skin for FW 2.1.x on FW 3.0.0, it appears cut off.
The way to create a skin for FW 3.0.0 is the following (I don't know if there are others):
- Make the skin using Skin Maker like if it was for FW 2.1.x (http://wiki.gp2x.org/wiki/Skins);
- Remember that Utility icon must be replaced with an Help icon;
- For icons without animation, canvas size of button.png (for imgmain & imgsetting) must be 480x720, and icons must be at the top of canvas, such as in this image:
I don't know because it needs this trick, but it works
However, after to have modified canvas size, preview in Skin Maker will be warped, then it's better to do it at the end. Finally, pressing F8 skin'll be exported in .gps format as usual and suited for FW 3.0.0.