At the mement, whenever I open a file, either from nautilus file browser, or from the CLI, the file opens as a new tab in the current window, rather than opening a new window.

I have “open_files_in_new_window” set to true in the user settings, but this appears to be ineffective. Are others experiencing this issue as well or is this Ubuntu specific? Could this be related ot the fact that I have the Flatland theme installed?
Flatland theme: https://github.com/thinkpixellab/flatland