article layout

hi, I would add a new stile in all articles layout. I think I have to create a new class, but I don't know how to do

Could you be more specific ? You want to add new suffix ?

I would add a background image in all article,
now I do it with editor in every article, I wish it were default.
Where do I change the css?

You should look to default article view tmpl file, you can find it in template/html/com_content/article/default.php