Global syntax highlighting theme.

Maps token types to {color, [effects]} pairs. Colors are resolved through Pote's theme system, so they accept atoms, hex strings, or "theme:<key>" strings.

When rendering, the lookup order is:

1. Language-level colors (lang.colors)
2. Global Syntax.Theme
3. Hardcoded defaults

Example

%Theme{
  name: "dracula",
  colors: %{
    keyword:  {:pink,    [:bold]},
    string:   {:yellow,  []},
    comment:  {:cyan,    [:italic]},
    number:   {:purple,  []}
  }
}