That's a part of who the characters are, not a theme. It's motivation, not a huge part of the story.
No, hell no. Sephiroth is doing all of that for himself. He wanted to become a surpreme being with all of the power of the planet. Jenova be damned.
There's no theme of love. There are people who love other people in the game, but it's not a huge plot point therefore it's not a theme. A lot of the characters are very selfish, despise others, and do everything for themselves. There's no theme of greed or hate is there? Because that's a pivotal part of who a lot of the characters are.
The big theme of FF7 is life, and how sacred it is, and how quickly and harshly it can be taken away from us, and the pain it leaves behind. Which is why people should be trying to save the environment, and do everything to help each other that they can.