Dunder Mifflin Shirt | The Office

Try by making child theme.
https://codex.wordpress.org/Child_Themes

You can add a css rule to do it

.site-info a:after{
content:".";
}

The period will not be in normal data flow. It would show on your screen but you wouldn’t be able to select and copy it.

Sorry correction the above code will add an extra period after wordpress.com . Use this instead

.site-info a:first-child::after{
content:".";
}