Lost Galaxy had a somewhat animal theme and it was mostly about space. 

Though I'm thinking if we do have a theme, why not make it planetary? Or if that's too restricting, just space related. 

Ex. The Blue Ranger could  be a Comet. 

Yellow = Star 

Red = Sun  

Black = Void or Vacuum. 

So on and etc. 

I actually gave up cause I couldn't think of anything for the rest lol.