Unfortunately, we don't have this feature, despite really wanting it for some time. It's on our list, but It's not currently on our road map.
In the mean time, it is possible to use MS paint to invert the color of a screenshot.
You can also show your support for this feature on our feature voting page here: http://ideas.saleae.com/forums/214303-saleae-ideas/suggestions/8010369-user-interface-color
trello ID: 55f9a454e5c15646caef7c55