This sentence may seem obscure because the word "style" can take many different meaning according to the context, both in french and english.
In this quote, the word "style" clashes with the context of peace and war: we do not think as peace or war coming down to "style"... this is, at first sight, disturbing...
And then, we start to think. Peace, War, style...
What if Camus wants to give us, his reader, people of Peace, an idea of what it is to live in War? an advice in order to survive? give us hope? or not... ?
"To be born to create, to love, to win at game is to be born to live at peace."
This is the original state. Basically, we are all born to live at peace, that's what we are when we are born, when we grow up as child, and as adults, when our environnement is at peace. Unless we are depressed, we all have a positive attitude to life when we are at peace.
"But war teaches us to lose everything and become what we were not."
War induces us to change. We learn to adapt to constraints induced by war, and in effet war changes us all.
"It all becomes a question of style".
The meaning of "style" becomes all important here, Camus tells us that "style" is the key to understand what changes in us, fundamentally.
What meaning can "style" take?
At least 15 different meanings. 4 or 5 compatible with the quote context. I prefer the fourth one.
"4. A quality of imagination and individuality expressed in one's actions and tastes:"
Style is a quality, a perception of our individuality, a perception of who we are. A perception of who we are by other, but it can also be a perception of who we are by ourself.
Going from Peace to War changes style, our style. it changes who we are so deeply that it alter the perception of our individuality by other, and by ourself.
This is my interpretation, but feel free to try other meanings.
Camus has managed to pack his impression, his feelings of what it is to go from Peace to War into a simple, but immensely rich and revealing, expression: "A question of style".