Overlay for background images
The easiest way to create a colored overlay for your image and slides is using the Background color feature and lowering the image's opacity.
You can use the same features for setting the overlay for videos, just lower the opacity at the Video instead.
Background overlay using a static overlay
If you want to create a background overlay using a static overlay you should add an area layer in Canvas Mode, set the semi-transparent color you want to have. The area layer should be 100% wide and high.
Then move this area layer below the content layer.