Installing a new theme for Gallrey 3 is a pretty straightforward procedure. Here are the steps you need to accomplish:

1. Download the desired theme package locally for your computer.
2. Extract it locally on your computer.
3. Use an FTP client to upload the theme folder to your gallery installation “themes” directory.
4. Login to the Gallery 3 admin backend.
5. Go to Appearance -> Theme Choice and enable the newly uploaded theme.