Summary

• Add two new CSS preview themes — Google Docs and Gmail — so users can
  draft Markdown and copy-paste rendered content into Google Docs or Gmail with
  minimal formatting cleanup
• CSS-only additions to the Styles folder (folder reference) — no Objective-C
  code changes or Xcode project modifications needed

Google Docs theme

Matches Google Docs default formatting for seamless copy-paste:

• Arial 11pt, 1.15 line-height, 816px max-width (~letter page)
• Heading sizes matching Docs defaults (H1=20pt, H2=16pt, H3=14pt, H4=12pt)
• Link color #1155cc with underline (Google's standard hyperlink blue)
• Code blocks: grey background (#e8eaed) with green text (#1e8e3e),
  matching the Google Docs code add-on style
• Prism syntax highlighting overridden to uniform green so the preview
  matches what appears after pasting into Google Docs
• Blockquotes with 3px #cccccc left border

Gmail theme

Matches Gmail compose/reading defaults:

• Arial 10pt, 1.5 line-height, 600px max-width (email body width)
• Conservative heading scale for email context
• Web-safe fonts only (Courier New for code — no Roboto Mono)
• Gmail quoted-reply style blockquotes (2px #cccccc left border)
• Light #cccccc table borders (Gmail strips heavy styling)

References

Both files include header comments documenting color/layout choices with source URLs:

• Gmail CSS Support (official)
• Gmail supported CSS properties
• Gmail rendering guide

Test plan

• Build MacDown and confirm both "Google Docs" and "Gmail" appear in
  Preferences > Rendering > CSS dropdown
• Open a Markdown file with headings, lists, links, code blocks, tables,
  and blockquotes
• Select "Google Docs" theme — verify preview renders with Arial, tight
  line spacing, letter-width layout, green code blocks
• Select "Gmail" theme — verify preview renders with smaller font, wider
  line spacing, narrower email-width layout
• Copy rendered preview content and paste into Google Docs — verify
  minimal formatting differences
• Copy rendered preview content and paste into Gmail compose — verify
  minimal formatting differences
• Verify existing themes are unaffected
• All existing unit tests pass

Closes #347