I noticed that high CTR usually goes with high PPC keywords.
I manually edit high traffic domains with high PPC (top earners) to fit with the domain name as much as possible and make it look more like a website. If you have hundreds of domain names (if not thousands) to manually modify each theme to fit the domain as much as possible is a full time job. If I have a domain name that starts to increase in traffic suddenly I try to focus on those to optimize keywords as much as I can and theme. You might want to leave the rest on out to make as much variation as possible so that which ever ones randomly start to get higher CTR float out to the top and now you can make them even better, if that fails change back...
Honestly themes make a big difference from my experience. Some visitors are really searching hard for whatever they need even if they know the are on a parked page and not a real website, usually they would end up on your parked page when they exhausted their search engine queries. So any website link related to their needs is important to them. However, usually these people got a bunch of windows opened at the same time, and if your website fails to catch their eye, they might just move on to another one that stands out. This process really takes off when a searcher already spent hours behind the computer looking for something and their brain is already working reflexes. Keep this in mind when you set your keywords and themes for domains with traffic. Sometimes I add a picture of lets say "old people" or "young women" as a theme to target specific groups when they are in their "prime" conversion period.
Also, I noticed using "new/modern real estate" pictures works well with many keywords. I target this specific theme because we are in a recession from a real estate market crash, I find it that when people see pictures of what they saw mostly in advertisements back in early 2000s CTR increases, I think it is because their brain tends to relax a bit when presented with a good memory. This works well for a divorced person who lost their home and family when they remember their life a few years ago (subconsciously). A relaxed brain is better a focusing and possibly this makes them take more risk instead of just sticking to their usual brands and closing the page when they recognize a parked domain name.
So real estate and women (with matching colors) would work for just about anything.
