one way is to download ubuntu tweak from internet, just google it, once you installed it extract the zip you downloaded from here to the folder ./home/(your-user-name)/.themes
once you finished that launch ubuntu tweak from the applications menu and you can change the theme from there. To change the window border you will have to install dconf-tools. open up the terminal and paste this line sudo apt-get install dconf-tools* once it finishes launch it from the apps menu and navigate trough org.pantheon.desktop.gala.appearance on the theme box type harvey and then claick enter and your done. i wish it was easier.
on the launchpad page of Tom Beckmann (one of the main dev of elementary there is already a elementary-dark-theme)...it need just some fix to get perfect.. for example there are some problems with audience.. you may try to take a look at it and maybe use it to finish your project