Is it possible to change the layout of a theme?

When using your templates, are the designs changeable? For instance, can I remove the envelope design from a contact form?

See attachement

Yes, it is possible to change a theme once it is applied, this needs to be done injecting your custom CSS codes, this guide will help you with that: How-to-Inject-Custom-CSS-Codes

The code needed to remove the background should be this: 

.form-all {

    background: none;

    height: auto;

} 

The form should display like this: 

Hope this helps.