There are likely some CSS properties being inherited from other stylesheets. You just need to customize the CSS for the plugin.
Here is some information that may help you with customizing the CSS forms:
http://butlerblog.com/2011/07/25/customize-the-wp-members-stylesheet/
You only need to specify the location of the custom stylesheet in the plugin settings. But, if you’re up to it, a more elegant way of loading a custom stylesheet is wp_enqueue_style:
http://butlerblog.com/2012/02/20/loading-custom-stylesheets-with-wp_enqueue_style/
There are three stylesheets included in the download – the default, and two others. This should give you a variety to start from. Ideally, you would create your own stylesheet to fully integrate with your theme, but these give you a good starting framework:
http://butlerblog.com/2012/02/19/new-css-stylesheets-included-in-wp-members/
Lastly, when tracking down styles, I highly recommend Firebug (getfirebug.com). It’s a free browser plugin and allows you to right-click and “inspect element” which will show you what properties are applied to a given element and where they are coming from (good for tracking down inherited properties from other stylesheets).