Tweak style of Filament markdown previews

[JSn1nj4]

The original purpose of this ticket was to build my own Markdown preview—as in, a full "live" preview to see what the content will look like. This is mostly unnecessary since Filament's supplied markdown editor does such a good job at styling the Markdown text directly without doing a full to-HTML render.

However, the color scheme is unfit for dark mode. So this ticket is now about tweaking the Markdown editor's color scheme—at least for dark mode.

If there can't be separate color schemes for dark and light mode, a scheme that works better for both should be selected instead.

[JSn1nj4]

This is much lower priority now with Filament's markdown rendering. It's almost as good as having a true preview mode, just because it styles the markup inline.