Is it possible to style a border of a "beamercolorbox"? Like in the theme Szeged, but with ONE thin line above the footer instead of the two thick lines.
The theme Szeged uses the miniframes outer theme. Additionally, it sets the color of th separation line:
It's the parent color for all separation lines. They could be changed indivudually. To remove an individual line, set its background color to empty. For example, this removes the line below the footer, like desired:
Actually, those lines are beamer color boxes themselves. They are defined within the miniframes outer theme this way:
Here you can see the thickness of the line. If you would like to make it thinner, like your question shows, you could modify that template, such as:
Here you can see, the line below the footer is removed and the line above the footer is replace by a thin line. At the top the original lines can be seen.