Adds backgroundColor parameter to override Theme's default

[Kinark]

There's no way to use a custom theme in Hilightr if you use it as a module, which usually isn't a problem because there's a bunch of themes, but backgroundColor is the only one that should be easily overridable, because it's the only color that may need to be full custom to match custom interfaces, and since it's something we can do without messing with Hilightr pkg, we should do it.

This PR adds a parameter to the View to do that.