Implement theme customization with preset and custom color schemes

[saismrutiranjan18]

Provide theme options for the editor and preview to match user preferences.

Preset Themes:

Editor Themes:

• Light (default)
• Dark
• Sepia/Cream
• High Contrast
• Solarized Light/Dark
• Dracula
• Nord
• Monokai
• GitHub Dark/Light

Preview Themes:

• Match editor theme
• GitHub markdown style
• Medium-style article view
• Documentation theme
• Minimal black & white

Customization Options:

• Custom background color
• Custom text color
• Custom accent/link color
• Font family selection (mono, sans, serif)
• Font size adjustment
• Line height adjustment

Implementation:

• Theme selector in settings/toolbar
• CSS custom properties for theming
• Save preference to localStorage
• Respect system theme preference (prefers-color-scheme)
• Export/import custom themes as JSON

UI Elements:

• Theme picker modal with live preview
• Quick theme toggle button (moon/sun icon)
• "Create custom theme" option
• Share custom themes via URL parameters

[cassidoo]

Thank you for this one! I was planning something just like this with a toolbar for it. I'll be sure to tag in this issue once implemented.