The site branding elements in the header of Twenty Seventeen are left aligned. The logo, site title & description can be centred within the header.

The default header logo, site title & description.

The header logo, site title & description centred within the header.

Where and how to center the logo, site title & decription:

In WordPress Admin > Appearance > Edit CSS.

Enter the CSS code below.

The CSS code:

#masthead .wrap {
    /* position: relative; */
	text-align: center;
}