Transparent Background
Viewport widget supports transparent background which means you can render actors on top of other elements without a solid background.
To use this feature however you must first enable alpha support in post process via Project Settings -> Rendering section.
Preview​
The setting was experimental in 5.4 but not from 5.5 and above. In addition, 5.4 required restarting the editor and since 5.5, restarting the editor is not required.
5.4 or previous​
Go to Project Settings -> Rendering section and under Post Process category, change Enable alpha channel support in post process to Allow through tonemapper.
After changing, restart the editor.
5.5 and above​
Go to Project Settings -> Rendering section and under Default Settings category, set Alpha Output to true. Restarting the editor is not required.
If the aforementioned setting is not enabled, a message will be displayed on the UMG widget.
Above message can be safely ignored if you don't plan to use alpha feature. It is nothing but a friendly reminder.
To actually see the transparent feature it is best to render isolated actors which you can do through Isolate Actors node. Here is an example setup:
