CSS change for form button in Membership plugin

Hi,

My form button in the Membership plugin looks a bit weird compared to how it should actually look in my theme.

As you see here:
http://imageshack.us/a/img197/5339/new2co.jpg

The button is with rounded corners, my theme is all squared corners, this type of button does not exist. I tried deleting the entire button.css file in the membership/css folder, but it still doesn't correct itself after my theme css.

If I copy the code: <p><input type="submit" value="Register My Account »" class="alignright button blue" name="register"></p>

That is used in the Membership plugin, into a page from my theme, I get the correct look for the button like this:
http://imageshack.us/a/img132/8567/buttonnik.jpg

The membership plugin somehow adds these rounded borders that messes up the layout of the button.

How do I stop this?