The default blender theme is located within blender's source code, it is the values used unless a preset theme is used to override them.
The theme settings used at startup are contained within your startup.blend file, you can make any adjustments you like and Save User Settings to have those theme settings active when you start blender.
If you want to create a theme and share it with others, you can create an xml file that contains the theme settings, that others can then use. To start with blender's default theme, click Reset to Default Theme, make any adjustments you like and then click the + next to the theme list and type in a name to use for your new theme. The xml file created for this can be found under scripts/presets/interface_theme/ either with your blender install or within your configuration folder. If you make adjustments, you can click the + again and enter the same name to update the xml file.
You can find the xml files for the themes that are included with blender inside scripts/presets/interface_theme or browse them online here, there are also some more themes in the contrib repository.