Having the plugin add the CSS needed to do that in a way that works with every theme would be tricky. However, since widgets are always displayed in the same manner regardless of theme, it might be possible. I’ll look into adding some basic widget styling options in the next release.
For now, the best way to style the widgets would be to add whatever CSS rules you want to your theme’s style.css file. To style the widgets you can use the unique id given to every widget (Ex: spw_widget-3) or the “spw_widget” class given to all widgets generated by Simple PW Ads.
If you give a link to a page on your site that’s showing an ad, I can suggest some CSS that would center it.
