Here's a simple theme I call "Purple".
To install, download the attached file and copy it to the "Themes" folder under the location where you installed the game. For example, if you installed in "\Storage\Program Files\Smoresoft Sudoku" folder, create a "Themes" subfolder at "\Storage\Program Files\Smoresoft Sudoku\Themes" and copy the attached file ("Purple.theme") to the new folder.
I'll eventually make the installation easier, but I want to release an early sample theme in the hopes that someone more artistic will create something cool.
