Dark Mode: code block is not dark in note preview

When using dark mode, the preview note option in the web app shows a nice dark color scheme for code blocks. The same does not happen when previewing the note from the Android app instead.

### Expected
To see code blocks in a dark color scheme like we do in the web app:

### Observed
Code blocks are not changed at all:

### Reproduced
1. Add a code block to a note
2. Change the theme to dark
3. Choose to preview the note

#### Internal reference: p1616480324000200-slack-C0133BTEDLJ

Reproduce the issue in the Android app by creating a note with a code block, switching to dark theme, and opening note preview. Compare the Android preview with the web app; done when code blocks use an appropriate dark color scheme in Android preview.

Written by the indexing model from the issue text.