You can't re-texture a star (yet, I think)
You can re-texture planets, however. For this reason the system must be in .xml file.
Put the texture in the Media folder in the Universe Sandbox folders in Documents.
Then, open the .xml from the Systems file from the Universe Sandbox folder with Notepad (i.e. click on it and place it on the blank Notepad).
Change the two Appearances into Texture, then put the texture name (i.e. earth-like planet.png, gas giant.jpg) in between.
Finally, check it in either US1 or 2 and see the system.