How do I save a theme so that I can share it.
If you create a new theme it will be located in the blender scripts/presets/interface_theme folder. The location is dependent on your operating system.
If you hover your mouse of the theme name in the User Preferences / Themes panel dropdown list it will tell you its location in the tool tip
Attachments
Thank you 
I modified one of the presets but when I close blender it doesn’t save the changes