JSON Editor supports Syntax Coloring :

It can be customized with preferences with Window / Preferences and JSON / JSON Files / Editor / Syntax Coloring page :

If you select Dark Theme :

By default the JSON Editor will adapt it to this theme :

This feature uses the following CSS css/dark/preferencestyle.css

JSON comments doesn't exists, but the JSON Editor support comments by using :

• /* and */
• or //

Here a sample with JSHint :