One possible theme could be discovery. Since it's the 'first story', it might be about discovering something new in the home, like a hidden attic or a secret passage.
The theme of identity could also be present. The home is a place where one forms their self - concept. How one views themselves in relation to their home, whether they feel a sense of belonging or not, can determine the path they take in life. If a person has a strong sense of identity rooted in their home, they may strive to uphold the values associated with it in their journey towards their destiny.
My first story at home could be about the time I learned to ride a bike in the driveway. I was so excited and nervous at the same time. My dad was there to hold the back of the bike, and when he finally let go, I felt like I could fly. It was a great moment of independence and achievement for me.
One possible theme could be self - discovery. Discovering one's sexual orientation and coming to terms with it is a big part of such a story.
One possible theme could be friendship. Animations often revolve around characters forming bonds and helping each other out.
Nature and its beauty could be a major theme. Geese are part of nature, and the story could be about the wonder and awe of experiencing a goose in its natural habitat for the first time. It could also touch on themes of growth, as in the growth of understanding about the natural world through this encounter with the goose. Also, the story might carry a theme of simplicity, highlighting how a simple encounter with a goose can have a profound impact on a person.
One possible theme is self - acceptance. Discovering one's homosexuality and learning to be okay with it. Another could be the theme of love, the first time falling in love with someone of the same gender. Also, the theme of courage might be present, as it often takes courage to face the world when realizing and accepting one's gay identity.
One possible theme could be loss. Since it's about a first lover who is now like a shadow, it might be about the loss of that relationship and the feelings associated with it.
One possible theme could be growth. Learning to ride a bike is a step towards independence and growing up. Another theme might be discovery. Discovering the world around while on the bike, new places, new friends perhaps. And also, the theme of perseverance as it surely took some falls and getting back up to master riding the bike.
One possible theme could be discovery. Since it's about the 'first' story, it might be about the discovery of the black rail itself, whether it's a discovery of its existence, its unique features, or its habitat.
One possible theme is the sense of belonging. Home is where one feels they belong, and the story might explore how characters find or create that sense of belonging within their home environment.