Here is a simple step by step tutorial on how to create and register a theme in Sitefinity.
First go to: Administration > Settings
Then click on : Advanced
Next go to: Appearance > Frontend themes.
Then, name your new theme and enter the path.
If you don't know the path you can copy the path of another theme in your project and replace the last part:
Now you can create the theme, got to: Administration > File structure.
Next, navigate to your App_Themes folder:
Create a new folder, and name your theme.
Inside your new theme folder create ...