Change Default Table Style when opening a CSV file
I'm not aware of the possibility of changing the default, but a useful workflow is: 


set up the doc with the exactly the styles you need, 
'Save as template' to the template chooser,
open Numbers
new doc with your new template
drag the CSV onto the table


Actually pretty quick to execute (after initial template configuration) and very flexible (allows different styles for different purposes). In combination Launchbar (etc), this is pretty seamless and doesn't involve any messing about with positioning windows, etc.