What is the best thing to use to clean (dust) dark brown leather furniture?
I just use a lightly dampened microfiber cloth. I then wipe it with a dry microfiber cloth to catch any dust or link that was missed by the damp one.
I dust mine regularly with a Swiffer duster then occasionally use Mother's leather cleaner/conditioner. It is in a red spray bottle in the automotive section at Walmart. It smells wonderful and works great.
I use a damp microfiber on mine as well and occasionally will use a PH-balanced cleaner on them once or twice a year. After any type of cleaning, be sure to condition them with a leather conditioner (like the one mentioned above). The worst thing for leather is for it to dry out. The conditioner keeps the leather supple and makes future cleaning easier.
For leather, simply wiping the furniture down with a dry cotton cloth in routine is the basic care.