I am not sure it belongs to the plugin or the theme.
Well both of these are obviously plugin issues as frontier theme does not come with a “registration form” or a “slider” on the content area.
1. You should see your slider plugin settings or ask your plugin’s author. I can’t think of anything on the theme that would affect your slider’s images.
2. This is also a plugin issue though I can see that one of the containers of your form has a set height of 378px. I doubt that this come default with your plugin. It’s possible that you might have entered this on your plugin’s custom user css if there is any. Removing this
height: 378px; fixes your issue with the buttons.