I've hardly been using Tweetdeck before, as I did not really need
the kind of scalability it provided, so I first took some time to get used to Tweetdeck...
Tweetdeck enhances the capability of Twitter by a proper UI, offering several different columns, for you to easily watch several different aspects of Twitter, like mentions, trends or activities.
Now Cashmere, in a nutshell, is a CSS style script, modifying the visuals of Tweetdeck.
Scripts like this exist for several different websites and can provide a completely different look than the original site.
To properly use Cashmere a browser-plugin like Stylish has to be installed. The instructions for this can be found on the projects GitHub page. The release versions of Cashmere (at the time of testing 1.1.0) can be grabbed there as well.
Installing is pretty easy, though it might be a little misleading. In newer versions of Stylish you can simply go to the site you want to modify and generate a script automatically, but all in all it's pretty straight forward.
Once installed you can visit TweetDeck and by reloading you should immediately see that the script is active, as the whole background of the page should have been replaced. (This can be customized in the css sheet)
If you have any issues with that, welcome in the dirty swap of browser-compatibility...- I've first wanted to test this on my every-day Firefox browser first, but some modifications through css were not allowed, and the script at first didn't even run. It seems that Stylish for Firefox requires a slightly different syntax, therefor this will not be a very long nor detailed review, as I only tested it a few hours yesterday evening (TweetDeck going down didn't really help...).
As I mentioned this to the author he recommended Chrome, which worked flawless out of the box. The theme itself (providing a dark and a light version) is pleasing to the eye and provides a nice alternative to the default visuals. Furthermore I really want to point out, that the Project is very clean and proper structured. (As this project is the successor of modern-tweetdeck I was able to look at the structure of the old project - and the new code really looks a lot cleaner and generally better arranged.)
To sum it up Cashmere is a well designed alternative to the default TweetDeck look, but you should be aware of incompatibility as you have to see for yourself whether Cashmere works on your system or not. A proper testing of different browser would be quite nice, but for Chrome this is definitely a recommendation.
Edit: After releasing this article the author immediately assured me to improve cross-browser compatibility and provide an easier setup for Firefox. Really looking forward to the future of Cashmere !