chore(docs): add styling customization guide for email editor

[joaopcm]

Summary

Adds a new Styling page to the editor docs covering how to customize the editor's appearance with CSS.

Stacked on top of #3098.

What's covered

• CSS variables — Full reference of all --re-* custom properties (colors, dimensions) with light/dark defaults
• Dark mode — Both prefers-color-scheme and class-based (.dark) approaches
• Data attribute selectors — Complete reference for every data-re-* attribute across all UI components:
  • Bubble menu ([data-re-bubble-menu], items, groups, separator)
  • Node selector ([data-re-node-selector], trigger, content, items)
  • Link selector ([data-re-link-selector], form, input, apply, unlink)
  • Link bubble menu ([data-re-link-bm])
  • Button bubble menu ([data-re-btn-bm])
  • Image bubble menu ([data-re-img-bm])
  • Slash command ([data-re-slash-command], items, categories, empty state)
• Editor content classes — .tiptap selectors for typography, layout (.node-columns, .node-section), buttons (.node-button), links (.node-link), alignment attributes, and column data types
• Practical examples — Custom brand colors, custom bubble menu item styles, custom slash command appearance, styling editor content

Also adds a "Styling" card to the overview page.

Test plan

• Visit http://localhost:3000/editor/features/styling
• Verify all CSS variable tables render correctly
• Verify code blocks with CSS selectors are readable
• Check the Styling card appears on /editor/overview

🤖 Generated with Claude Code

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Summary by cubic

Adds a new Styling guide to the editor docs for customizing the editor with CSS variables, data attributes, and content classes. Also adds a “Styling” card to the editor overview.

• New Features
  • Reference of --re-* variables with light/dark defaults, plus prefers-color-scheme and .dark support.
  • Complete data-re-* selector map for bubble menus, node/link selectors, slash command, and image/button menus.
  • .tiptap selectors for typography, layout, alignment, and column types.
  • Practical CSS examples for brand colors and UI tweaks.

^{Written for commit 681056f. Summary will update on new commits.}

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Project	Deployment	Actions	Updated (UTC)
react-email	Ready	Preview, Comment	Mar 20, 2026 8:07pm
react-email-demo	Ready	Preview, Comment	Mar 20, 2026 8:07pm

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Latest commit: 681056f

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[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.