A short while ago I ported the contributed module Photoswipe to Backdrop from its Drupal 7 version. This module uses an open source jQuery library by Dmitri Semenov to implement a gallery of images which can be swiped one by one, either to the left or to the right. With a touchscreen device this is a normal swiping action; without a touchscreen small right and left arrows can be used.
Backdrop has a new basic theme called, unsurprisingly, Basis, and I have been trying it out with, in my view, some considerable success. It produces excellent results 'out of the box' but, more importantly to me, it provides a very satisfactory foundation for producing a customised theme that gives a distinctive appearance to a website.
I have just completed the transition of a well-established website from Drupal 7 to Backdrop. Although, as explained below, quite a significant amount of data has been involved, the main effort has been to rethink the design of the site from a fixed layout to one that is responsive and is much more suited to smartphones and iPads, while still looking good on more traditional screens.