Develop a musical composition using the outlined alphabetical depiction of the structure and the provided theme.
X:1
L:1/8
M:6/8
K:Bb
['B6 B6 d3 d3 f3 d3 c>cccBc']  
X:1
L:1/8
M:6/8
K:Bb
 F | FFFFDF | FBF F2 B | BBBBFB | d6 | B3 z2 E | EEEEEG | _A>AA G2 B | BBBBGB | BeB B2 B | BBBBBB | 
 A3 c2 F | F3 F2 F | _GF=E F2 A | c3 z3 | B6 | B6 | d3 d3 | f3 d3 | c>cccBc | d4- d z | B6 | B6 | 
 d3 d3 | f3 d3 | c>cccBc | d4- d z |]  