tables in scribus are a work in progress.
if the layout is compatible with tabs, you should probably use tabs instead.
you should be able to remove use the "none" color, and -- at least theoretically -- apply it to the whole table... but it's always a big try and error before you get there.
if you really need tables, it's probably better to create them in libreoffice and import than as pdf or svg into scribus.
... but, in most cases, people prefer charts to tables : - )