Added optional Background color and opacity to YAPA 1.0 theme

[alexis-]

Hello, I've made the following modifications:

• Added: Background color to YAPA 1.0 theme
• Added: Design time model to YAPA 1.0 theme for design-time typing & validation
• Misc: Cleaned up a little bit (e.g. unused declarations)

I've also applied my automatic default coding style to the files I was editing since none was provided with the project. Sorry if that's inconvenient, the editor kept mismatching the formats every time I typed something.

[alexis-]

Looks like the AppVeyor errors are generated by issues outside the scope of this PR changes.

[floatas]

Tried to run this branch, looks like application is invisible for some reason, tried changing theme from dark to white, but same issue. App is not visible and not clickable.