Customising colors and styles

Listing as **good-to-have feature** .
You can 👍 this issue to increase priority. Please let me know your opinion about listed options in comment.

### Option 1: Config file
Define your own design by defining colors (name or hex) for pre-defined items.
For example:
- Project list
- Task list (Pending, Today, Overdue)
- Task detail title

### Option 2: Themes
Design can be changes using a theme file.
Pre-defined themes if possible (monokai, solarized, darcula, matrix, etc)