Split style scheme GSettings into light and dark

Right now we have one style scheme GSetting that we manually change every time you use the light or dark buttons in the style switcher. Instead, we could have a light GSetting and dark GSetting, and just toggle between them with the buttons.

- [ ] Change the UI to just say, "Light" and "Dark" instead of anything about "Solarized" OR programmatically grab the name of the relevant style scheme
- [ ] Create the new GSettings
- [ ] Change the scheme based on the GSetting
- [ ] Make sure we still load the correct one at runtime

This makes it a little easier for downstreams to set their own default light and dark color schemes if they want.

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首先跟踪现有的 style switcher 及其当前的 style-scheme GSettings 键。检查 Light 和 Dark 按钮如何更新 scheme，以及应用程序如何在运行时加载它。完成的标准是：Light 和 Dark 的独立设置能够正确切换，UI 使用预期的标签或 scheme 名称，并且下游能够设置相互独立的默认值。

由索引模型根据 Issue 内容生成。