[UWP] Allow using XAML Brush theme resources for host config color options

Today, hosts have to manually change their host config when the theme (accent color) or high contrast changes, since colors are all specified with absolute values in the host config.

	"containerStyles": {
		"default": {
			"foregroundColors": {
				"default": {
					"default": "#FF333333",
					"subtle": "#EE333333"
				},
				"accent": {
					"default": "#FF2E89FC",
					"subtle": "#882E89FC"
				},

Instead, it would be much easier for UWP hosts if they could just provide a XAML theme resource brush, which automatically updates when themes change, which is what apps use in their own normal UI, like seen below…

<TextBlock Foreground="{ThemeResource AccentBrush}"/>

The host config could be modified to be the following

	"containerStyles": {
		"default": {
			"foregroundColors": {
				"default": {
					"default": "#FF333333",
					"subtle": "#EE333333",
					"uwp": {
						"default.themeresource": "NormalBrush",
						"subtle.themeresource": "SubtleBrush"
					}
				},
				"accent": {
					"default": "#FF2E89FC",
					"subtle": "#882E89FC",
					"uwp": {
						"default.themeresource": "AccentBrush",
						"subtle.themeresource": "AccentSubtleBrush"
					}
				},

And if the uwp.*.themeresource value is specified, the UWP renderer assigns the theme resource rather than the absolute color value.

[mdtauk]

I think for sure it would be something specific to the UWP rendering of a card, as it relies on being able to insert values provided either by the System’s settings/config or resources stored within the App’s resources.

So how you implement that in the HostConfig spec, would probably require a prefix. If it is kept to UWP resources only, then a uwp:default type value, which is ignored by other renderers, could be done. Or broaden it out slightly. system:default where each renderer can either provide a value or assign it from system provided colours. iOS has a set of standard colours in its UIKit API I believe. I can’t speak to Android’s API.

[dclaux]

I’m not sure this needs to be an official, documented feature. It breaks the portability of HostConfig which is the reason HostConfig exists in the first place.

I get why this is convenient and it seems to me that it’s OK to do it, but I think it should remain a UWP-renderer-specific feature. Every platform has its own model for theme colors; I don’t think we want to end up having all platforms’ models explicitly supported in HostConfig.

Also, I would go one step further:

"containerStyles": {
	"default": {
		"foregroundColors": {
			"default": {
				"default": "#FF333333",
				"subtle": "#EE333333",
				"uwp": {
					"default": "ThemeResource(NormalBrush)",
					"subtle": "ThemeResource(SubtleBrush)"
				}
			},
			"accent": {
				"default": "#FF2E89FC",
				"subtle": "#882E89FC",
				"uwp": {
					"default": "ThemeResource(AccentBrush)",
					"subtle": "ThemeResource(AccentSubtleBrush)"
				}
			},

This way you decouple the property from its format. Otherwise you might end up with multiple “default.XXX” in the long run.