I want to insert multiple 'markers' or thick black lines in the Backlog (with label), so i can visually seperate groups of Stories (typically between sprints)

Thanks for the feedback.

Themes are already visually separated by the grey total bars and the white space beneath each.   

We are considering adding a collapse story feature, which may help with your "tucking away" idea.

Unfortunately it's quite difficult to see exactly what we could to address your problem, however we have scheduled in a design review of the entire interface so we will take on board your feedback.

Thanks,

Matt