This commit completely removes the iostreams.NewColorScheme() initializer function in favor of exporting the type fields for greater clarity in its use.
The result being code specifying only the fields that matter to test cases.
This commit refactors the color format around table headers to ensure the GitHub CLI uses thematically appropriate colors based on dark background, light background, or no color at all.
In order to do so, `ColorScheme` needs information from the terminal about the background appearance (dark, light, none) to determine appropriate muted color.
