1//condition: is the number gt 0
2const number = -2;
3
4<span className={number > 0 ? "css-positive-number" : "css-negative-number"}>
5    {number}
6</span>

Summary: Using the ES6 ternary operator, read the className attribute like this:
"If number is greater than 0 then use "css-positive-number" class, else use "css-negative-number" class."