[Bug]: Dark mode only themes the editor, not the Markdown preview pane

[taylorarndt]

What happened?

When dark mode is enabled, only the left side (the editor) picks up the dark theme. The Markdown/HTML side preview pane on the right stays light — it keeps rendering on a white background with dark text, so the split view is half dark, half bright.

For low-vision and light-sensitive users this defeats the purpose of dark mode: the bright preview pane is exactly the glare dark mode is meant to remove.

Expected

When dark mode is on, the side preview should render with the same dark theme as the editor (dark background, light text), so both panes match.

Steps to reproduce

1. Open a Markdown document so the side-by-side preview appears on the right.
2. Enable dark mode.
3. Observe: the editor (left) is dark, but the preview (right) is still light.

Likely cause

The side preview is a separate WebView that renders render_preview_body(...) HTML. The editor theming is applied to the wx editor control, but the preview HTML/CSS doesn't receive a dark stylesheet (or a prefers-color-scheme / theme class), so it always renders light.

Notes

• Affects the in-app side preview (SidePreview); worth confirming whether the modal MarkdownPreviewDialog has the same gap.
• Contrast in the dark preview must still meet WCAG AA (4.5:1 text).

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