I got a list of themes from someone else a long time ago. It's probably needs to be updated to include more, but here's what it has: Snow/Ice, Mountain, Hills, Cave (Dirt), Water, Slime, Factory, Space, Night, Hills, Garden, Lava, Beach, Sand Cave, Sky, Ruins, Carnival, Metal Warehouse, Lakeside, Forest, Rock Cave, Desert, High Caves, Icy Mountain, Machinery Hill, Fire Valley, Castle, Abstract.
If you run out of ideas, try fusing two of the themes together, or just look around on the internet or the world around you for inspiration. Another thing you could do is look through the different flats and textures that are in srb2.srb and think of ways you could use them in a level.