Download the file or install with Package Decontrol:
How to install in TextMate?
I don't know but it might be something like:
How to install manually (tested in Sublime Text 3)
- Click Preferences -> Browse Packages...
- Create a new folder called "Grandson-of-Obsidian" (or whatever you'd prefer it be named)
- Copy the GrandsonOfObsidian.tmTheme to that folder
- Click Preferences -> Color Scheme -> Grandson-of-Obsidian -> GrandsonOfObsidian.tmTheme