'Messages' shows chat bubbles in a rectangular box of the same color
Since I have replaced my iMac's HDD and used a Time Machine backup to restore my data, the chat bubbles in the Messages app are surrounded by a rectangular box of the same color as the chat bubbles. They have the same size because of which I assume it's an issue with the transparency beyond the chat bubble.
Also, when clicking on a chat bubble, the rectangular box as well as the chat bubble get selected. Changing colors of the chat bubbles just changes the color of the rectangular box. Restarting Messages, restarting the iMac and even resetting the PRAM haven't led to any change. How can this issue be solved? It's kind of driving me crazy.
P.S.: Before the backup everything was normal.
iMac (20-inch Mid 2007), OS X Mountain Lion (10.8.2), 2GHz, 4GB RAM