When I create a site there's a lot of white space around a section. That's the only section and I'd like to have a tiny margin.
Is it possible to change the margin/padding?
It helps to distinguish between classic and new Google Sites when asking questions, as the answer is not always the same for each.
Assuming you mean new Google Sites then as far as your question is concerned, the answer is no.
New Google Sites is pretty good at automatically configuring the output for different sizes, and orientation, of screens to support monitors, tablets, and phones. The trade off for that is that you loose some control over how the content is displayed; including margins and padding.
You can embed your own HTML, and have far more control of what happens with that section of display. But not outside of it.