@SempervivumMay 04.2019 — #How can I ensure that the three first items not change color when hovered?[/quote]The easiest way is assigning a class to the elements to be hovered. Is there a better way to do this?[/quote]Fine so far. I guess that the purpose of the empty container is to arrange the first two items to the left and the other ones to the right. This can also be achieved by applying margin-right:auto to the logo container. No auxiliary container needed.
This link shows some advanced techniques of arranging flex items:
@SempervivumMay 05.2019 — #I'm not shure what kind of effect you intend to achieve. At the right (about and magnifier) the image is very dark and a background darker would not be visible. One can simulate such effect by applying a black background and a low opacity: