Changing theme forces exit from markdown preview

### Expected
Changing theme should not affect the selected note view mode

### Observed
If markdown preview is active, changing theme will force the switch to edit mode

### Reproduced
1. Open any note in markdown preview mode
2. Change app theme ~> app will switch to note edit mode.

https://user-images.githubusercontent.com/73365754/107233108-e0f11a00-6a2a-11eb-9a32-3694686e162d.mov

Make|Model|macOS Version|App Version
-|-|-|-
Mac|MBP 16|11.2|2.8 (11218)

Reproduce the issue in the macOS app by opening a note in markdown preview mode and changing the theme. Trace the theme-change handling and note view-mode state; done means the selected markdown preview mode remains active after the theme changes.

Written by the indexing model from the issue text.