Powershell: Enable color themes to improve contrast for accessibility

[SteveL-MSFT]

ContrastAnalyser determines if the color contrast is sufficient to make text legible for accessibility

For example, the default error message colors on Windows, the red foreground on the black background is insufficient and should be a brighter red according to this tool. Should be able to define custom colors for Progress, Error, Verbose, etc... and package it as a theme.

[Jaykul]

We can't hard-code things like that in PowerShell, @chuanjiao10 -- those settings depend on your terminal.

[bitcrazed]

@SteveL-MSFT A thought on this: As you know, in FCU, we updated the default console palette to provide better contrast on modern displays. PLUS, we shipped a new tool to allow users to load color "schemes" if they prefer (esp. useful to those who're color-blind/sensitive).

If PowerShell was to fix its palette use to use color[0] as background, etc. rather than color[5], it'd be able to enjoy the palette improvements we've made ;)

[SteveL-MSFT]

@bitcrazed Changing Windows PowerShell to use color[0] is a non-starter for various reasons. For PowerShell Core 6, we're using color[0] as background color.

[SteveL-MSFT]

The new ANSI rendering proposal will allow setting color definitions which should satisfy this