Hi there. From the module menu (the little down arrow by the module name), you can go to Settings > Opt In Email Module Settings, then click on Stylesheet where you can make the changes you want.
Keep in mind, though, that v3.0 is rather old and may have conflicts with the new DNN 5.6. Please consider upgrading to Opt In Email 4.3 to make sure you have all the new features, enhancements and fixes.
http://www.datasprings.com/products...t-in-email Thanks!
Candace