Feature Proposal: Theme Tag

Themes need some TLC... Maybe the ability to define a theme inside a tag will help something along the lines:

```

colors.background: red;

```
or maybe json would be better?
```

{
colors: {
background: "red"
}
}

```
HOCON could allow both syntaxes: https://github.com/akkadotnet/HOCON

The API will be updated to allow a theme update in one string:

```
IStyled.AddTheme("MyTheme", themeJsonOrString);
```

Lastly, need to add a way to switch between themes. API:

```
IStyled.SetTheme("MyTheme");
```

Not sure about a Tag version for that. Maybe:

```