Theories are just that theories.
Croat Y-haplourgroup DNA:
I2 ~ 40% (balkan)
R1a ~ 23% (eurasian, associated with Eastern Slavs, but also Iranians, and other eurasians, etc.)
E ~ 10% (west asian, or possible north african)
R1b ~ 6-7% (west european, associated with germans, french, english, irish, etc)
J1 ~ 6-7% (middle east semites, aka jews and arabs)
I1 ~ 5-8% (scandis)
So they are as much germans as they are jews and arabs or swedes.
They are more Slavs then Germans.
P.s. In 1940's Croats collaborated with Nazi's. So they wanted badly to "be of German origin".