Background Effects are a new feature that has been added to ProPresenter in ProPresenter 7.11. These are a great addition to the ProPresenter Editor that can expand creativity by taking slide objects and adding a gaussian blur to the slide object over the media that is triggered to the Media Layer, or the Live Video Input Layer, as well as the ability to enable a color Invert for the Media Layer, or the Live Video Input Layer.
Creating this effect can be done in the Presentation Editor, or the Theme Editor by adding an object to a slide, and changing the "Fill" of this to "Background Blur" or "Background Invert". This changes the fill (within the Editor, and the thumbnail) to show the shape of the object as a crosshatch. If the "Background Blur" effect has been chosen, you can adjust the Saturation and Blur of the object fill.
These Effects can be stacked as well to create some really neat looks. In the example below, we've stacked the Background Invert, and the Background Blur to have an outlined look to a blur.
More about Themes can be found in this article.