I know that my addition to the post is quite a bit late/dated, but I just wanted to add my 2 cents.
Rather than approach a specific theme built into the quiz, why not simply create the quiz to reflect the Moodle site's theme? I get that inline css styling is inefficient, inconvenient, and limiting, but a simple template that follows a plan for colors, layout, etc. goes a long long way. Moreover, there is a TON of flexibility if one is familiar with using the Cloze Question syntax as it allows for a number of input formats and variations of how answers are displayed (e.g., horizontal, vertical, shuffled, etc.).
But this generally means that a) someone has to know to do basic html and css, b) they know what environment (i.e. desktop/mobile) to design for, c) know the syntax editor, and d) consistently apply the design.
Where I work, none of the teachers would be able to any of the above; rather they would stick with simple quiz creation which is completely functional but not necessarily aesthetic. Students are given time to do these on site in a computer lab with rather nice monitors, etc. They're also a bit too long to really want to do comfortably on a phone though a tablet would be fine.
But since people were asking for pictures, I figured I'd share some examples of ones we have. Below is in the Moodle App and it doesn't quite display the headers properly (they're <h2>) but its fine on a regular browser. As as mentioned earlier, each browser displays radio buttons (etc) differently.
I'm not making any argument that these examples "look good", only that they they are generally consistent and have nothing to do with the actual theme. A simple style was made and applied-even many old, particularly poor looking tests, were re-made with it.