rat-salsa主题支持

这将主题分为两部分，

• Scheme - 基础颜色方案，有足够的颜色可以进行操作。

• A concrete DarkTheme that takes that scheme and produces Styles for widgets.

  This intentionally doesn't adhere to any trait, just provides some baselines for each widget type. You can use this as is, or copy it and adapt it for your applications needs.

  In the end I think this will be just some building-blocks for an application defined theme. I think most applications will need more semantics than just 'some table', 'some list'.