Sony has released a tool to allow for easy creation of Xperia themes for Androids running their version of the operating system. This application installs on your computer, and when used in conjunction with an image editor (Photoshop is recommended and well-supported) allows you to customize over 300 different assets. Using custom icons, buttons and backgrounds it's now pretty easy to create your very own theme for Xperia phones and tablets.
The tool is still in beta, so expect a few issues here and there, but Sony has set up a support forum to address the inevitable questions. To learn more about the tools and for installation instructions, visit Sony's developer site.