I've found an older map with a nice-looking grid frame.
How can I create this with the QGIS Print Composer (also programmatically with Python)? I didn't found anything to control the line style of the grid frame.
There is the possibility to duplicate the grid frame style of the topographic map (see the comment to LaughUs answer). Logically you can't use "Move item content" with another map and a rectangle on top. You have to uncheck these two elements first to move within your map and then you have to adapt the extents.
I'm wondering why there is an option to change the line style of the grid and not for the frame. Until now I've never used a special line style for the grid lines (multiple lines, etc.) for my GIS work.