Good news for designers, you can create your own XMB theme. SCEJ released a program to do that.
Step 1: You will need to download the official[Register or Login to view links] from Sony's Japanese Web site.
Step 2: Extract the file to a folder on your computer.
Step 3: Open command prompt and navigate to the folder.
Step 4: The folder should contain a sample folder, a GrimConv folder, a p3tcompiler.exe and other assorted files. To create your theme you will need to run the p3tcompiler.exe passing in the sample.xml.
Example: If you are in the p3tcompiler_v101 folder, you run: p3tcompiler .sample1sample.xml
Step 5: This creates a number of .gim files and a sample.p3t files in the sample /01/ directory. Take the sample.p3t file and transfer it to a memory stick in the following location: PS3->THEME
Step 6: Plug the memory stick into your PS3 and select the theme setting, where you will be given an option to install; do so, and you will have your sample theme.
Step 7: To construct your own custom theme, it's a case of editing all of the .png files in the sample directory, and re-run all of the above.