[Preferences] Implement Syntax Color Scheme Selection

Created by: ian-james

Implement on either the preference page or the feedback page the ability to change the color scheme for the syntax highlighting. This will improve usability and accessibility.

Prefer this to only be available via the preferences page as the feedback page upper section is quite full. Additionally, it's unlikely to change to frequently once a preference has been set.

We must make sure the user can get back to the feedback after going to preferences.

• add other syntax highlight colour schemes (related: #37 (closed), mentioned in: #29 (closed))