Improve recognizability of inline code elements

[dcalhoun]

Fix

Fixes #1891. Increase recognizability of inline code elements in Markdown
previews by applying a subtle background color to them.

Before	After	After (Dark)

Test

1. Create a new note.
2. Click note Info button in the top-right.
3. Enable Markdown formatting.
4. Add an inline code block to the note content.
5. Click Preview button in the top-right.

Expected Outcome: The inline code element is styled with a monospace typeface
and a background color different than rest of the content.

Release

Improve recognizability of inline code elements.

[sandymcfadden]

Straightforward, works well and makes it much nicer to read inline code.