An enhancement to Simple, the default SilverStripe theme by Sara Tušar Suhadolc. This is not a full featured theme because it depends on it.
Check if your SilverStripe installation has Simple installed: under the themes/ directory there must be a "simple" folder. If not, grab the theme in the way you prefer and install it.
To install the treeish extension:
unpack the tarball under themes/ (or perform equivalents steps via git)
cd themes wget http://dev.entidi.com/p/silverstripe-treeish/source/download/master/ -O tarball.zip unzip tarball.zip
rename the base directory to "simple_Page"
mv silverstripe-treeish-master simple_Page
That's it. At the end, your themes directory must contain the folders simple and simple_Page.