Style core forum for Busy theme

Even without advanced Forum, the core forum should look at least bearable.
Following the sample of Corolla, it has to be styled according to the base design. Though explicit screens for the Forum are missing, this should be doable.

If a designer feels fit to do so, provide a mockup for the looks of the forum first. But we can go without, Should though see if we have table headers defined and the core icons fit. If not so, we need to do some icons for this (though not many, joergsart already provided a baloon icon and an add icon.