An updated manual covering all the mod commands available at this time has been prepared. I'm not sure where it is right now or when it'll be released.
The GUI modding is pretty simple. One command modifies the default background and border for the windows in the game, which is pretty much everything except for the main map area and the scrolling battlefield used in the main menu/game setup area. The other modifies the window background/border for individual nations. I haven't had time to do more than test to make sure it works, but it works just fine. Some really nice effects should be possible, as well as some mind-bendingly awful ones.
Here are the summaries of the commands. The first should go in the mod's opening section, the second in an individual nation-modding block.
#defaultguiframe “<.tga file>”
The default GUI background for the game may be replaced with this command. The file to be used should be 512 pixels wide by 128 pixels high and will be used for all new pop-up windows and so forth. If there is a #guiframe specified in a nation’s definition, the file listed there will override the default one. The guiborder2.tga located in the game’s root directory can be used as a guideline for sizing, etc.
#guiframe “<.tga file>”
The default GUI background for the game may be overridden on a national level by using this command. The file to be used should be 512 pixels wide by 128 pixels high and will be used for all new pop-up windows and so forth. The guiborder2.tga located in the game’s root directory can be used as a guideline for sizing, etc.