[Plugin: WP-Members] "Submit" Button on Registration Forms Grayed Out

Sounds like you have some outside CSS that is being inherited. All you need to do is add some CSS definitions for #wpmem_reg .buttons. You could do that in your child stylesheet or use a custom stylesheet for the plugin (see: http://butlerblog.com/2011/07/25/customize-the-wp-members-stylesheet/)

Alternatively, if you had some more complex changes (and depending on your knowledge of php), there is a filter for the generated html of the form: wpmem_register_form so you could use php’s str_replace in your filter to take out the generated html of the existing button and put in your own).