I sat down this evening to design some levels. I've taken a number of different approaches including a random generator - at least, a tool that gives me ideas to turn into levels, and SVG guides that help me design the jumps.
A few levels in I had the idea of picking up the base colour of the rocks from the fill colour of the SVG paths, so I can now add light and shade to the rocks right in Inkscape, where I'm also positioning the flies and water. This improves the look significantly with very little time spent.
The game already reloads the level from disk when you retry a level, so this has also made level editing pretty fast - at least, it lets me set the atmosphere and layout for a level quickly. I think the look still needs improvement, and I need to sort out my backgrounds, but there may be time over the next 2 days.