Constant BG_COLOR 

pub const BG_COLOR: Type = 5;

The resolved background color of the cell (GhosttyColorRgb). Flattens the three possible sources: content-tag bg_color_rgb, content-tag bg_color_palette (looked up in the palette), or the style’s bg_color. Returns GHOSTTY_INVALID_VALUE if the cell has no background color, in which case the caller should use whatever default background color it wants (e.g. the terminal background).