Copy your theme into the following directory:

administrator/components/com_hydra/themes

Before we can use our new theme, we must tell Project Fork that it exists. Do do so, open your database tool (phpMyAdmin).

Select your database and then the table "#__hydra_theme"*.
Click on Insert to add a row of information.

Leave the field "theme_id" emtpy.

"name" should be the name of the theme-folder  - "my_theme_folder" for example.

"Label" is the name of your theme - That's how you want to call it. "My cool new theme" for example
And finally click go. Have fun with your new theme!

*"#__" represents the Joomla table-prefix which is by default "jos_".