I've been watching the anime and I'm noticing a large number of nautical references, from the logo to the music and others which have slipped my mind. The logo itself features the tops of the letters forming a wave-like pattern and the top of the "T" resembling an anchor as well as the final "L" have a tail which appears to be in the shape of a fish-hook. This hook shape also appears in the guilds logo along with a harpoon. As for music I cannot help but hear the repetition of the can can music as well as once what sounded like "land of hope and glory". If anyone has any information on why there is the inclusion of such things I would graciously appreciate it. I may however be hallucinating in which case I'm sorry to have wasted your time
I think you're hallucinating
Considering the HUGE cast size, the various other logos and the 20+ opening songs of different styles, and some variations of the logo, it's hard to say they have a single theme.
The logo itself, btw, it just a fairy with a tail. This picture of a different version of the logo seems to help people with seeing it.
But if I have some time later, i'll check out the music and such to see if i can find some themes between them. Let me know if you have any more specifics that point towards that theme.