feat: add themes list command to CLI for theme discovery

- Added `markitect themes` command to list all available themes
- Supports multiple output formats: table (default), list, and json
- Allows filtering by theme scope: ui, document, branding, or all
- Shows key properties for each theme (colors, fonts, etc.)
- Includes legacy theme mappings and usage examples
- Enhances discoverability of the layered theme system

Usage examples:
- markitect themes
- markitect themes --format json
- markitect themes --scope ui
- markitect themes --scope document --format list

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

markitect/plugins/builtin/markdown_commands.py

@@ -1708,6 +1708,7 @@ class MarkdownCommandsPlugin(CommandPlugin):
             'md-get': md_get_command,
             'md-list': md_list_command,
             'md-render': md_render_command,
+            'themes': themes_list_command,
             'md-index': md_index_command,
             'md-explode': md_explode_command,
             'md-implode': md_implode_command,
@@ -1959,6 +1960,104 @@ def md_render_command(ctx, input_file, output, theme, css, edit, editor_theme,
         raise click.Abort()
 
 
+@click.command()
+@click.option('--format', type=click.Choice(['table', 'list', 'json']), default='table',
+              help='Output format: table (default), list, or json')
+@click.option('--scope', type=click.Choice(['ui', 'document', 'branding', 'all']), default='all',
+              help='Filter themes by scope: ui, document, branding, or all (default)')
+def themes_list_command(format, scope):
+    """
+    List all available themes and their properties.
+
+    Shows the available themes that can be used with md-render and other commands.
+    Themes can be used individually or combined in layers.
+
+    Examples:
+        markitect themes list
+        markitect themes list --format json
+        markitect themes list --scope ui
+        markitect themes list --scope document --format list
+    """
+    from tabulate import tabulate
+    import json
+
+    # Get theme data
+    layered_themes = []
+    legacy_mappings = []
+
+    # Process layered themes
+    for theme_name, theme_data in LAYERED_THEMES.items():
+        theme_scope = theme_data['scope']
+        if scope == 'all' or scope == theme_scope:
+            properties = theme_data['properties']
+            # Get key properties for display
+            key_props = []
+            if 'body_background' in properties:
+                key_props.append(f"bg:{properties['body_background']}")
+            if 'font_family' in properties:
+                family = properties['font_family'].split(',')[0].strip().strip('"\'')
+                key_props.append(f"font:{family}")
+            if 'link_color' in properties:
+                key_props.append(f"links:{properties['link_color']}")
+            if 'accent_color' in properties:
+                key_props.append(f"accent:{properties['accent_color']}")
+
+            layered_themes.append({
+                'name': theme_name,
+                'scope': theme_scope,
+                'properties': ', '.join(key_props) if key_props else 'default styling'
+            })
+
+    # Process legacy mappings
+    for legacy_name, expanded_themes in LEGACY_THEME_MAPPING.items():
+        legacy_mappings.append({
+            'name': legacy_name,
+            'expands_to': ' + '.join(expanded_themes)
+        })
+
+    if format == 'json':
+        # JSON output
+        output_data = {
+            'layered_themes': layered_themes,
+            'legacy_mappings': legacy_mappings,
+            'usage': {
+                'single': 'markitect md-render file.md --theme dark',
+                'layered': 'markitect md-render file.md --theme dark,academic',
+                'legacy': 'markitect md-render file.md --theme github'
+            }
+        }
+        click.echo(json.dumps(output_data, indent=2))
+
+    elif format == 'list':
+        # Simple list output
+        click.echo("Available themes:")
+        for theme in layered_themes:
+            click.echo(f"  {theme['name']} ({theme['scope']})")
+        if legacy_mappings:
+            click.echo("\nLegacy mappings:")
+            for mapping in legacy_mappings:
+                click.echo(f"  {mapping['name']} -> {mapping['expands_to']}")
+
+    else:  # table format (default)
+        # Table output
+        if layered_themes:
+            click.echo("Layered themes (can be combined):")
+            headers = ['Theme', 'Scope', 'Key Properties']
+            table_data = [[t['name'], t['scope'], t['properties']] for t in layered_themes]
+            click.echo(tabulate(table_data, headers=headers, tablefmt='grid'))
+
+        if legacy_mappings:
+            click.echo("\nLegacy theme mappings:")
+            headers = ['Legacy Name', 'Expands To']
+            table_data = [[m['name'], m['expands_to']] for m in legacy_mappings]
+            click.echo(tabulate(table_data, headers=headers, tablefmt='grid'))
+
+        click.echo("\nUsage examples:")
+        click.echo("  Single theme:    markitect md-render file.md --theme dark")
+        click.echo("  Layered themes:  markitect md-render file.md --theme dark,academic")
+        click.echo("  Legacy mapping:  markitect md-render file.md --theme github")
+
+
 @click.command()
 @click.argument('directory', type=click.Path(exists=True, file_okay=False, dir_okay=True))
 @click.option('--output', '-o', type=click.Path(),