feat: add dark PDF theme

[VOIDXAI]

Summary

• add --theme support with light as the default and dark as an opt-in
• switch stylesheet, Mermaid config, and Mermaid background together for dark-mode PDF output
• document the new CLI/front matter option and add regression coverage for theme selection

Closes #1

[VOIDXAI]

Review summary:\n- implementation matches issue #1 and cleanly threads theme selection through CLI, front matter, stylesheet, Mermaid config, and background rendering\n- regression coverage looks good, especially CLI override vs front matter\n- documentation is updated and CI is green\n\nNo blockers from my side; merging this now.