You're right, this is inconsistent. Syntax colours and styles are exported with their defaults, while editor and Markdown preview colours currently include only values you've changed.
Missing values use Studio's built-in defaults when imported, so the exported file isn't a complete snapshot of your theme.
I've logged this as a bug. I'll update the export to include all supported colours for both light and dark themes, including unchanged defaults. Existing theme files will remain compatible.