Gallery themes can be downloaded from here:

http://codex.gallery2.org/index.php/Gallery2:User_Contributions#Themes

Once you’ve downloaded the theme, you will need to upload it and unpack it under your /gallery2/themes folder.If you have downloaded a .zip file, you would need to unzip(extract) it and upload the unzipped folder that contains the theme to the /gallery2/themes folder. The folder that contains the theme will have the files “theme.inc” and “theme.css” in it.

Follow these steps to modify the theme:

STEP 1 Login at your admin area by clicking on the Login button and filling up your admin login details.

STEP 2 Click on [Site Admin].

STEP 3 Click on [Display – Themes].

STEP 4 Click on the [All Themes] tab and install the one you have just uploaded by clicking on the link.

YOU HAVE SUCCESSFULLY INSTALLED A NEW THEME IN YOUR GALLERY!

You can set the new theme as default if you would like to use it on your website.