Short answer: Forge no longer has a Backdrop panel. It used to style the area behind and around the stage with per-scene colors and gradients; that idea was retired in favor of something simpler. The space your background art doesn't cover is now always plain black - the same for every scene, with nothing to set up. What the panel was really for is handled by the Background event itself: switch it to a solid color when you want a flat color look, and use its Framing setting to decide how off-size art fills the frame. The reading interface - textbox, name box, choice buttons - is styled in the Theme view, same as before.
The space around your art is black
If a scene has no background yet, or its art doesn't cover the whole frame, the rest is black. There is no per-scene wash to author and no automatic gradient - a neutral black surround frames every scene for every reader.
Want a flat color? Use a Background event
A colored scene doesn't need an image file. Add a Background event and flip its source switch from Image to Solid color, then pick the color from the swatch or type an exact hex value. Because it's a real background, the color behaves like everything else in your story: it takes effect at that beat, it can change to another color or an image later, and it's part of the stage like any background art - handy for title cards and fade-to-black or fade-to-white moments.
Off-size art: the Framing setting
When a background image's shape doesn't match the stage frame, the Background event's Framing setting decides what happens:
- Stretch to frame - stretch the art to fill the frame exactly. This is the default.
- Fill frame (crop) - scale the art up, keeping its proportions, and crop what overflows.
- Fit inside (letterbox) - show the whole image, with black bars filling the rest.
Art that already matches the frame's shape looks identical under all three. The choice matters for off-size art - most often a Ren'Py import whose backgrounds were drawn for a different screen shape.
Backdrop vs background
These used to be different layers - a background was the picture on the stage, and the backdrop was a separate wash behind and around it. Only the background remains: it's the picture or color a Background event sets, and anywhere it doesn't cover is black.