Hi,
You need to assign a class to your form or a module class suffix to your module and add your css to that class. Edit your subscription module and go to the tabAdvanced. In case you want to center the entire module you add a module class suffic. When yiou need to center the form only anter a Form css class. (Please note that whenever you add classes in Joomla the first one needs to be preceeded with a space).
Once done add the class and it's css to your site's custom css.
.yourclass {text-align:center;}