Well, my guess is to replace the correct DDS files in the UI folder structure.
They seem to be DXT5 compressed DDS files. 512x512 with 3 mipmap levels. I've never made one before, so I don't know if those settings are allowed to be variable. At the least, they probably need to be a power of 2 in size.
If you use Photoshop you can use this plugin
... if not, I have no idea. Keep in mind that DDS files do not use color masks to define transparency but use a per-pixel alpha value. Probably listed as a "channel". So if you see magenta in the above images, it is just a matte, not an alpha mask.