Some style changes (header, colors, ...) (#136)

* - Invert header colors for better readability
- Increase size of header image and adjust layout
- Change font-color of sent date to grey because it is not as important as the content (with darker grey on hover)
- Use same opacity of delete button as in contacts app (hover: opacity: 1)

* Remove unnecessairy margin-right

* Use flex-box in header and adjust more to other apps