The ID is for the DVWP. There is no style that you can apply to a whole DVWP in one fell swoop. Inside the DVWP there will be lots of other CSS classes for the different elements and they will be applied, overriding anything, even if you could use the web part ID for a CSS format.
A DVWP typically displays in a table. There are different classes and styles for TH, TR, TD and so on. If you want to change the appearance, then you need to tackle these classes for the table elements.