Please note, I am not updating the script at the moment, but maybe Instapaper Threestyled  fits your needs.
Instapaper restyled in action

 

„Instapaper restyled“ is a userscript that applies a new interface to the Instapaper service. It was inspired by Helvetireader and Instapaper Fluid.

Okay, how do I use it?

Make sure you have Greasemonkey for Firefox installed or you‘re using another userscript supporting app like Fluid (Mac). Now direct your browser to the following userscript to install it:
Install Instapaper restyled userscript
Now visit the Instapaper website and enjoy! If you use a browser that doesn't support userscripts, or you want to customise it, download the CSS file.

May I download the fancy icon to use it with Fluid or Mozilla Prism?

Yes of course, you can download it here. The icon was originally designed by Mathieu Cailleaud, an awesome interface designer from France.

All by yourself?

No, as mentioned above, the script is heavily influenced by Jon Hicks (Helvetireader) and Tim Van Damme (Instapaper Fluid). Also I have to thank Michael Bayer for his JS & CSS tweaks and patient support.