Make theme customizable

[LucasPickering]

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It'd be nice to customize the color of the TUI.

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Configurable theme section of config.yml. To start with, it'll just have two fields:

theme:
  primary_color: yellow
  error_color: #0000ff

ratatui::Color deserializes from both HTML hex strings and color names automatically, so we don't even need to implement that.

We may also want to add a field secondary_color, which would be used for things like the template color. Could also punt that to the future though.

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We could make the entire style book customizable (Style does implement Deserialize!), but I think that would be way too complex and also opens us up to maintaining the entire set of styles as part of the external API. Making a few core settings customizable, then deriving styles from that, I think is a much better solution.

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If anyone wants to pick this up, the work involved is:

• Split Theme into two structs (theme.rs):
  • A small simple one that defines what's configurable in the config, which I would have keep the name Theme
  • A complex one with all the individual styles, which I would call Styles. This would be derived from the theme, and stored in TuiContext
• Add a new field theme: Theme to the config (config.rs)
• Document theme field in the config docs