Defines the behaviour for a theme module in Aurora.Uix.

A theme module is responsible for providing the CSS rules that are used to style the components. This module provides the __using__ macro to inject the necessary behaviour and callbacks into your theme module.

## Example

  defmodule MyApp.Theme do
    use Aurora.Uix.Templates.Theme

    def rule(:root) do
      """
      :root {
        --primary-color: #3b82f6;
        --secondary-color: #6b7280;
      }
      """
    end

    def rule(:button) do
      """
      .button {
        background-color: var(--primary-color);
        color: white;
        padding: 0.5rem 1rem;
        border-radius: 0.25rem;
      }
      """
    end
  end