Theme could be for instance a "parchment theme", as shown below.
Another theme could be an "earthlike theme".
A third theme could be something like this.
Now, even if you would have to select a theme, there will still be quite a few options to tweak, to create the kind of map you want, for that given theme. The exact specifics of what will be user preferences is yet to be decided, but I'm coding most things to be customizable.
If you already have themes in mind that you would find interesting and useful, - maybe something the internet is not already littered with, please let me know!
(The images linked here are not mine, I only wanted to display them as good examples of what I might want to do!)