I've installed Ecwid and it looks very weird. It doesn't look like on your demo.

Most likely, your site CSS styles affect Ecwid's look. The fastest way to fix this issue is to show Ecwid in a iframe.

The other way is to add some custom CSS styles.

For example, if you use the Atahualpa WordPress theme, please add the following CSS style to your store. It will fix the issue.

div.ecwid table td, div.ecwid table tr, div.ecwid table{
    margin: 0px;
    padding: 0px;
    border: none;
    outline: none;
}