{r, g, b} in theme throws color error when used in fill with opacity.

[vacarsu]

Scenic cannot validate a fill color with opacity when using a theme with an rgb color because it expects an rgba color.

graph
|> rectangle({100, 100}, fill: {theme.background, 100}

The above will error when the background is set to an rgb color. This is partially fixed by #252 as it extends the named colors palette so you don't have to use rgb colors in themes. However I think scenic should automatically convert {{r, g, b}, a} to {r, g, b, a}