Loading theme affects all open instances

[dnjackson]

When I load a theme, it seems to update the display of all open visualization instances. I don't recall this being the behavior, and I wonder if it's a regression. It's especially annoying when you have instances open from different models because then they really get messed up.

[nmacedo]

Looking at the code it seems that it was always the case (there's really just one instance loaded at all times, when you close the current instance it just re-loads the previous one with the current theme).

When themes are loaded from .thm files it should be easy to keep track of the .thm associated with each instance and reload when needed. Keeping track of theme customisations not stored in a .thm is going to be more complicated.