Good news – I figured it out. There are multiple elements that can be configured for the title background; in my case Color and Image. from the title object, access properties and work with the Title Background settings.
The trick is to use an image (in my case a white png) and to ALSO specify a background color that is the desired border color (in my case black) for when the learner resizes their window.
The combination of the two allows me to control the background on which the content displays (white) and the non-content border color (black) that is seen when the window is resized in a way that violates the aspect ratio.