Local (offline) previews of posts and pages are currently useless, they're not showing what the post is going to look on the user's site. Local preview is just showing a post HTML output with a very simple CSS style and looks almost like the view in the visual/block editor.

Local previews are only available on local drafts. Local drafts will be very rare soon as we're going to synchronize them as soon as the app can, and local drafts will exist only while the app is in offline.

Note: Rendering the theme locally is way out of scope (it needs more than just a CSS file).