Before You Write Css Code You must know Store Layout Structure , Store Has Two Layout :1.
on Column Layout: this Type layout is Default Structure Of Any website
As You See here from Top To Down The Structure is Header then Header Menu Then Body then Footer. This is Default Layout Of All Store maybe some store has The Second Layout Type Two Column.
2. Two Column Layout: This Layout Split The Store To Two part One to right and other to left .you must know the class used in structure To develop corrected Style
Most Store Themes Use one of this Layout Types If there any update or new layout will be described her,little of store make private layout of it Theme.
You Must know the Store theme layout if one of the two layout or special if not you must speak with store holder to know about that.