Improve headings on mobile

[phseiff]

On mobile, I currently use png headings. These should be improved sooner or later (as soon as there is time for cosmetics at hand) in the following ways:

• Include a darker and lighter version, so the adapt on whether the page is shown in dark- or light mode.
• Include a small shadow, so the edge between text and not text is smoother (I now know how to do this using GIMP -> layer -> color to transparency).

And whilst we're at it, a smooth shadow for the main header and the image next to it to make them look mightier might also be a neat idea.

[phseiff]

This'll be irrelevant once issue #19 is fixed.

[phseiff]

This is now solved due to issue #19 being implemented.