Make 'none' syntax highlighting theme truly a 'none' theme

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Right now the 'none' syntax highlighting theme still uses highlighted lines for presentation. We could significantly speed up diffs for users with that theme since it doesn't do any code highlighting:

1. No highlighting step
2. No blobs fetching
3. A LOT less bytes sent over the network

A 'none' dark mode theme is required for this to work properly after #470413 (closed).

The change should be straightforward to implement after !196464.