The drop between step 2 and 3 of your stats is the proof, although I immediately noticed the problem: People don't go "back" to make a sale, they go forward. When they click your demo button, they no longer have a call to action button to lead them to the sale. Recommendation: add a top bar that stays with them as they check out the demo. Regardless of how you solve this, it's worth losing sleep over it.
2) Changing the demo content (text) to be about the theme itself.
At least in the intro paragraph. I found it very disorienting after clicking the demo button.
3) Pick 3 selling points, not 9.
The first one should almost convince me by itself. Make them bigger and more visual. The rest are just extras. Make them look like extras.
4) Remember that you're selling design to designers.
There are beautiful Wordpress themes that are free. Yours looks ok, but not $30 ok. ;) TELL ME WHY I LOVE THIS, and tell me in the first sentence.
5) Screenshots suck. Use the image to show me or teach me something I need to understand about this product. Better yet, get a really nice background image and fill that baby blue area with it. Add some emotion, man!
6) By pitching it based on Underscores and Bootstrap you're immediately selling to a technical audience. In my experience, visual creatives are not particularly technical. Why is bootstrap good? What is Underscores? etc.
7) A/B test versions with and without the price in the buy button. You include the price when it makes the product more appealing. Yours is high. the A/B test will give you a very rough idea of how much effect that has on conversion.
8) Include the demo / buy links in the header of your documentation. Beside the logo, not in the top-right corner.
These are some very good advice. Thanks. In coming weekends, I will make improvements suggested by you and everybody else to improve both the offering and landing page.
4898 total unique visitors.
1567 clicked demo button.
87 clicked buy button (but not completed transaction)
72 checked documentation
28 clicked on screenshot
4 completed sale.