[Dark mode] Empty screen on post preview is missing the dark background

Expected behavior

Background should be dark.

Actual behavior

Background should is white, text is the same color (and thus, unredable).

Note: there are multiple empty pages, I haven't checked if they used the same layout/styles.

Steps to reproduce the behavior

Test 1:

• Set the app or device in dark mode.
• Set airplane mode on (to get the "no network" empty view).
• Open a post in the editor.
• Tap the "Preview" button.
• Notice the background color is white, the text is not visible ("No Network" empty view).

Test 2:

• Set the app or device in dark mode.
• Switch to a Jetpack site (preview unavailable for locally modified posts).
• Open an existing post in the editor, edit the post, don't update it.
• Tap the "Preview" button.
• Notice the background color is white, the text is not visible ("Preview unavailable" empty view).

Tested on [device], Android [version], WPAndroid [version]

Reproduce the post preview's "No Network" and "Preview unavailable" empty views with Android dark mode and the reported offline or Jetpack-site conditions. Locate the shared preview empty-view layout or styles, then verify that both views have a dark background and readable text in dark mode.