Chrome OS text editor with autosave

Question

I want an text editor app like this:

• Runs in its own window.
• Works on Chromebook.
• Works with files.
• Autosaves.
• Has a full-page interface mode.
• Has themes or at least has a dark-colored theme.
• Gratis.

There are various tab-based text-editors with autosave that either don't work with files or work with one file at a time.

Some apps that almost work

• Minimalist Markdown Editor

  • Works with files and autosaves.
  • Only split-screen interface.
  • The theme is locked in bright white (no themes).

• Caret

  • Works with files.
  • Full-page interface and themes.
  • Lacks autosave.

• Ain't

  • Autosaves
  • Confusing interface.
  • No themes.
  • Only works with ain't files.

• md everywhere

  • Works with files.
  • Full-page interface and themes.
  • Lacks autosave.