How do I change the letterbox background while running?

I know you can change the letterbox / project background color in the properties panel. But is there a way to change it via events?

Like say you changed to an interior layout and wanted the letterbox to be dark. And when you stepped out to an exterior layout change the color to the grass color. Or in a dungeon change it to match the stone walls etc.

I hope there's an easier method but I found a workaround.

At the start of each layout, I load a stylesheet. And the stylesheet contains the "html" and "body" background color of the layout I'd like. It's a bit cumbersome because I have to create a new stylesheet for each / most layouts.