Add DarkEmber color scheme

[nkristek]

Summary

This pull request adds a new color scheme to the existing ones. It uses the color palette from the Dark scheme, but changes the accent colors to variants of orange (from blue).

Please note: This pull request does not include a change to the documentation.

Motivation and Context

Especially orange fits pretty well for a dark theme and it increases the variety of available color options.

Details

• Add DarkEmber.xaml
• Add DarkEmberColorScheme property to ResourceLocator
• Modify demo to loop over existing color schemes when clicking the 'Change theme' button (was toggling before)

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have written/altered unit tests for the changes.
• [x] Only necessary files have been added/altered.

[benruehl]

Thank you for contributing again! :)

In general, additional color schemes are welcome. I am unsure if we should add this particular one though, because it changes the accent color only. Individual colors can be changed by developers easily already. If we add this color scheme, I am afraid that we will have to include more color schemes all being identical except having different accent colors.

I'd be interested in the opinions of others regarding this topic, though.

From a developer's perspective I'd like to keep the number of included color schemes low to reduce necessary testing efforts.

[Insire]

In my opinion this would fit better in a separate repository/ nuget package. Maybe something like adonis-ui-extensions?

While i'm not opposed to having more colors and styles out of the box, i'd prefer to have the option to stay with the 2 default themes and optin to more.

[nkristek]

Thanks for your feedback. After some consideration I will close this PR.