Tomorrow Nightish is a Textual theme based on the Tomorrow Night family of themes. You can find Textual at http://www.codeux.com/textual/ or on the App Store.
This theme was adapted from the Simplified Dark theme that ships with Textual.
Check out this git repo or copy the theme folder into ~/Library/Application Support/Textual/Styles in a sub-folder and it will be an available option in the Textual preferences pane.
cd ~/Library/Application Support/Textual/Styles
git clone git://github.com/lukeman/tomorrow-nightish-textual.git