I assume the leaves of the thunia turning brown is not much of a problem. As a matter of fact, the leaves should turn brown and ultimately fall off at this time of the year. I have been waiting for about 6 weeks for the same to happen to my (first) thunia but it still keeps going strong.
I read somewhere that they should be grown in a shady spot and watered heavily until the leaves fall off. They should then be kept dry until the new shoots appear in the spring. At this time they should be watered and fertilized heavily.