fix color resource names

[danwalmsley]

Consider that you want to implement a DarkTheme

currently we have a light theme with resources named Light and Dark

This means in your dark theme, you would end up assigning colors like Black to a variable named Light and a color like White to a variable named Dark.

UWP take the approach of naming colors Low, Mid, High, etc.

This makes the same change.

[wieslawsoltes]

To update wiki:

Breaking changes in 0.7 version release:

• Renamed theme resource from ThemeBorderLightColor to ThemeBorderLowColor.
• Renamed theme resource from ThemeControlLightColor to ThemeControlLowColor.
• Renamed theme resource from ThemeForegroundLightColor to ThemeForegroundLowColor.
• Renamed theme resource from ErrorLightColor to ErrorLowColor.
• Renamed theme resource from ThemeBorderLightBrush to ThemeBorderLowBrush.
• Renamed theme resource from ThemeControlLightBrush to ThemeControlLowBrush.
• Renamed theme resource from ThemeForegroundLightBrush to ThemeForegroundLowBrush.
• Renamed theme resource from ErrorLightBrush to ErrorLowBrush.
• Renamed theme resource from ThemeBorderDarkColor to ThemeBorderHighColor.
• Renamed theme resource from ThemeControlDarkColor to ThemeControlHighColor.
• Renamed theme resource from ThemeControlHighlightDarkColor to ThemeControlHighlightHighColor.
• Renamed theme resource from ThemeBorderDarkBrush to ThemeBorderHighBrush.
• Renamed theme resource from ThemeControlDarkBrush to ThemeControlHighBrush.
• Renamed theme resource from ThemeControlHighlightDarkBrush to ThemeControlHighlightHighBrush.