Class and inequality
The Greaser-Soc divide runs the whole novel. Greasers get jumped, arrested, and dismissed. Socs get protected by their wealth and status. Ponyboy keeps noticing that the divide is real and unfair, even when individual Socs turn out to be more complicated than the label.
Loyalty and belonging
The gang is the only stable thing most of these boys have. Loyalty keeps them alive and gives them identity. It also pulls them into violence they might otherwise avoid, and it destroys Dally when the person he's most loyal to dies.
Innocence and loss
The Frost poem frames the whole book: nothing gold can stay. Johnny and Ponyboy are still young enough to feel wonder, but the world around them keeps taking that away. The novel tracks exactly how innocence gets worn down.
Identity vs. stereotype
Ponyboy reads, writes, and notices sunsets. That doesn't fit what people expect from a Greaser. The book shows how labels flatten people and how hard it is to be seen as an individual when a group identity is stamped on you.
Grief and survival
By the end, Ponyboy has lost Johnny and Dally in quick succession. The novel doesn't skip over how that breaks him. Writing becomes the way he survives it, and the book itself is the proof that survival is possible.