How can I achieve the look that was created for the galleries in the "portfolio" section of the theme's demo? How do I create these galleries (including the square thumbnails and the "filters" that the demo shows? Is there documentation for this?
Unzip the file that you have downloaded from ThemeForest, look for a folder called Documentation, there you'll find all you need to know to set up Retro theme. In case you're still having troubles with the theme, open a new ticket. Cheers