I found a solution myself – had to set the z-index value in the frontend from 20 to 0:
.sitetitle {
line-height: 1;
color: black;
text-decoration: none;
white-space: nowrap;
z-index: 0; }
I did the same before in the custom CSS section without success. I'm not sure if this is the best way as the setting might be overwritten again when the theme is updated, but for now it works.
If there is a better way to do this, let me know..