Can anyone explain me that part in JSON please ?? (Wordpress Plugin)

This field you edited is not for HTML, it's the initial DataTable config, included in the code using CDN, as repository code

For customization, the plugin offers the possibility to add classes, as in this example with shortcode

[gdoc key="ABCDEFG" class="my-sheet" title="Tooltip text displayed on hover" summary="An example spreadsheet, with a summary."]This is the table's caption.[/gdoc]

See that the my-sheet class has been included, that you can put whatever you want and use it to stylize, as it will generate the html as

<table id="igsv-ABCDEFG" class="igsv-table my-sheet" title="Tooltip text displayed on hover" summary="An example spreadsheet, with a summary.">
    <caption>This is the table's caption.</caption>
    <!-- ...rest of table code using spreadsheet data here... -->
</table>

With this you create a rules in your css, for example:

.my-sheet{
    background-color: red;
}