Ship a companion MarpToPptx.Themes content NuGet package

Summary

MarpToPptx currently requires users to supply external CSS files via --theme-css or inline front-matter style:. There are no built-in or bundled themes — the theme: front-matter directive stores the name but doesn't resolve to anything without a matching CSS file.

We should ship a companion content NuGet package (MarpToPptx.Themes) that bundles popular community themes and our own DiagramForge-shared themes so users have a solid starter set out of the box.

Motivation

• Users shouldn't need to author CSS just to get a polished dark or branded deck.
• DiagramForge is rolling out shared themes (Cyberpunk, Synthwave) — a shared package keeps both projects in sync.
• Marp CLI ships built-in themes (default, gaia, uncover); we should offer parity.

Proposed Design

New project: MarpToPptx.Themes

• Content-only NuGet package that packs .css files as contentFiles so they land at a predictable path in consuming projects.
• Versions independently from the core MarpToPptx package.

CLI --theme <name> resolution

Add a new --theme <name> option to the CLI that resolves a theme name to a CSS file by searching (in order):

1. Current directory / relative path
2. NuGet content folder (from MarpToPptx.Themes)
3. User themes directory (~/.marp2pptx/themes/)
4. MARP2PPTX_THEMES environment variable path

Falls back to --theme-css <path> for fully custom CSS.

Initial theme set

Bundle the existing samples/themes/ collection plus the top-level sample themes:

Tasks

• Create src/MarpToPptx.Themes/ project with NuGet content packaging
• Copy/organize theme CSS files into the package with stable names
• Add --theme <name> CLI option with search-path resolution
• Add theme name resolution to MarpCompiler API for library consumers
• Update documentation (doc/using-templates.md, CLI --help, README)
• Add tests for theme resolution logic

Start by inspecting the existing CLI option handling, the MarpCompiler API, and the samples/themes/ collection. Define the new src/MarpToPptx.Themes/ project and trace how content files and theme names should be resolved across the listed search paths. Review doc/using-templates.md, README, CLI --help, and existing tests to establish the packaging, resolution, and documentation requirements.