In fact let me help you get started with some knowledge.
DDS is DirectDraw Surface. It is a DirectX Texture Map File (meaning image that wraps around a given object to map it's elements over it).
GIMP (GNU IMAGE MANIPULATION PROGRAM) supports editing DDS files and is completely free. It is somewhat tough to install for windows so if this is something you want, it's gotta be something you are willing to learn for. You will need to download a bit older version of Gimp for windows from back in 2012:
https://download.gimp.org/mirror/pub....8.0-setup.exe and once downloaded you'll need to get this plugin for it
https://code.google.com/archive/p/gimp-dds/downloads
Follow the instructions in the zip on how to install the plugin and then you can open DDS files. Which will let you edit these themes and change them to your hearts content.
Hope you have fun and learn a lot with this.