Add accessibility preference for background color
This has been requested many times. It was supposedly already fixed in version 3.0, according to [this issue](https://staruml.uservoice.com/forums/245359-feature-request/suggestions/14948286-diagram-background-color). But it is certainly not fixed in version 3.1.
For me it's an accessibility issue. Your otherwise excellent program is rendered (no pun intended) unusable, because 2/3 of the screen is a painfully glaring white.