Delta Air Lines flight 1253 was cleared direct to KRVUR to join the DADES1 arrival at FL 330. While flying direct to KRVUR, the flight crew said that Federal Aviation Administration (FAA) Air Traffic Control (ATC) gave the flight permission to descend to FL 310. As the airplane was descending, it approached some clouds that were visible in the moon light, and the captain activated the seat belt sign, and made a brief announcement as a precaution. The flight crew said that they then requested that ATC allow them to deviate to the left to avoid the highest cloud peaks that they could see, however at no time did the weather radar indicate any significant moisture returns. The flight crew said they were unable to avoid entering the clouds and the flight briefly encountered “moderate” turbulence. As the flight exited those cloud tops, they then saw more clouds ahead, along with some indirect lightning nearby. The captain made an announcement to the cabin for the flight attendants (FAs) to take their jump seats, and about a minute later they entered the next set of cloud tops and experienced brief, but severe turbulence. The first officer (FO) said that the turbulence encounter lasted about 10 to 15 seconds with the altitude deviating a little, and the airplane rolling about 10 to 15 degrees. He also added that the auto throttle system, increased the thrust setting, which required that they reduce it manually. After the turbulence event, the flight crew stated that they received a call from the cabin crew notifying them that a FA was injured. The injured FA stated that after completing cabin service they stowed the beverage carts in the back galley and within one minute of stowing the carts, they encountered “very severe” and unexpected turbulence. He said that the seatbelt sign was off, and he could not sit in the jump seats closest to him because they were all occupied by other flight attendants. He added that the flight was full and there were no passenger seats available as well, and the only available jump seats were far away. He said that as he reached to grab a handle near the carts he was suddenly thrown by the turbulence. He hit his head on the ceiling and came down on his right foot, resulting in a broken fibula. After the turbulence ended the flight crew gathered more information from the cabin and made a pilot report (PIREP) to ATC indicating that there was an injured FA aboard and requested that emergency medical services personnel meet the airplane at the gate. The first officer added that from his perspective, there was no indication of possible turbulence in the vicinity, and if it wasn't for the moon light making the clouds visible, the seat belt sign would most likely have not been activated, because the radar showed no reflection of moisture in the clouds.