Develop a musical piece using the given theme and incorporating the letter-based structure provided.
X:1
L:1/8
M:4/4
K:D
['', 'fg a2 (af) gbag fefa gfef dcde', '', 'a2 (af)']  
X:1
L:1/8
M:4/4
K:D
"^A" |] fg | a2 (af) gbag | fefa gfef | dcde fdec | dedc |1 AG :|2 A2 |]"^B" |] FG | ABcA dcAG | 
 FEFG ABcA | dcdf edce | d2 dd |1 de :|2 d2 |]"^C" |: A2 | dcde fefg | a2 (af) defg | 
 a2 (af) ggag | f2 (fe) de |]  