Add GitHub Tomorrow dark preview theme #106

Pull Request

Based on PR #1289 from MacDownApp/macdown by @elsiehupp
MacDownApp/macdown#1289

Description

"GitHub Tomorrow" is a preview-pane theme that combines the basic formatting of the GitHub preview-pane theme with the colors of the Tomorrow edit-pane theme.

Problem

There is currently no dark-mode preview-pane theme that looks nice with the Tomorrow+ edit-pane theme.

Solution

Create a new preview theme that:

• Uses GitHub formatting structure
• Uses Tomorrow color scheme
• Has a slightly lighter preview-pane background than the edit-pane to provide contrast
• Works well with Tomorrow-themed inline code blocks

Benefits

• Better dark mode experience for users
• Consistent color scheme between editor and preview panes
• Enhanced readability in low-light conditions
• Popular color scheme (Tomorrow/Tomorrow Night)

Implementation Notes

• Add new CSS theme file for preview pane
• Theme selector in preferences
• Test with various Markdown elements (code blocks, headers, lists, etc.)
• Ensure contrast is sufficient for accessibility
• Verify it works with syntax highlighting

Priority

MEDIUM - User experience enhancement

Labels

enhancement, ui, themes