Difference to the Snail theme is obviously the different design with transparent elements and a colorful background image. Also the behaviour of the Main div changed a bit. It now floats to the left and not to the right, so there is no huge gap between the left widget bar and the main content.
Note from the original theme: It has the bar on the left, but the widgets are on the right, because it's only the right bar moved on the left… So put the widgets in the right bar! Enjoy it!!!
The original Snail theme was tested with FlatPress 1.2 and 1.3, so I think this theme will work with 1.3 as well, as I haven't made that much changes in there.
Have a look into the Git as well: https://git.la10cy.net/DeltaLima/flatpress-theme-rainbow
Visit my Webiste :) https://deltalima.org
Please head over to the Forum post in the “Theme support” section