Center Table

Use the following CSS:

.thn_post_wrap table td, .org_comment table td {
    padding: 10px;
    text-align: center;
}

Don’t edit the theme files directly, otherwise your changes will be overwritten whenever the theme is updated.

A custom CSS editor (usually titled Additional CSS) is included in the Customizer as of WordPress 4.7 specifically for this purpose.

As an alternative, especially if you intend to modify more than just CSS, create a child theme.