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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