I think you answered your own question.
Start with a style and modify it.
If you have a css file that kinda sorta works already add in the missing properties. Unless it was designed for ubb7.x there will be many properties that don't match up due to naming conventions.
Many properties have a copy link while in the style editor.
You could try them, it will try to fill in some default settings then go back and tweak them once you figure out what is affected by the property.
