A theme as a document (spec/04 §5): stored and shipped as a value, and read back through every gate.

• the map form is %{version: v, theme: node} (to_map/1), and a store's fragment %{theme: node} (fragment/1), a library entry of kind :theme, reads as the same document;
• the JSON form is {"$version": v, "theme": {…}} (to_json/1), each color value a tag ($color, $series, $ramp, the transforms by name, $blend, $contrast) and pinned steps $steps.

Reading (from_map/2, from_json/2) runs the kernel's gates (spec/01 §5) — the version, the limits, the migration — then reads the form, then: a node that is a name reads as the built-in theme of that name, any other name is {:unknown_theme, name}; a variable anywhere is {:unresolved, :var, name}, since nothing applies a theme; a key the theme does not have is :unknown_key; and every contract a ramp, color or palette entry breaks is {:contract, message} at [:theme, :ramps | :colors | :styles, name].

As a fragment type (spec/02 §1), ramps, colors, styles and defaults are :union name maps and every other key :override, so a theme document stacks over a built-in as Macaw.Layers.Theme.new/1 merges.

iex> theme = Macaw.Layers.Theme.new(name: :house, axis: "#555555")
iex> Macaw.Layers.Theme.Document.from_map(Macaw.Layers.Theme.Document.to_map(theme))
{:ok, theme}
iex> Macaw.Layers.Theme.Document.from_map(%{theme: :dark})
{:ok, Macaw.Layers.Theme.dark()}

Ported from visualize's Visualize.Chart.Theme.