You're welcome, Neverwish--it turns out wishes do sometimes come true! I had your request in mind, particularly for the background image support, when I wrote the feature.
Let me know if you have specific questions about creating a theme. I'll post instructions at some point, but I won't have time to get to it for a few days. Hopefully most of the parameters in the theme file are self-explanatory.
Also let me know if you'd like more things to be customizable. I may not be able to do them all, but I have a few more parameters that I haven't exposed for themes yet, like adding padding between boxes and removing the outer border like some other Sudoku apps.
If you really want to get fancy, you can hide the gridlines and include a custom grid image instead. You'd have to create a separate image for each screen size you wanted to support, but I think it's fine to have themes that support a limited number of screen sizes.
For the next release, I'm also planning to have themes include sounds, so you can customize sounds for entering a digit, completing a box/row/column, completing the grid successfully, etc.
Looking forward to your themes!