Gutenberg is meant to be the common interaction point for making and customizing your website’s elements — navigation, content, site elements. It is not meant to take agency away from themes. Themes will continue to provide templates, designs and layouts. Blocks only make these things more portable and easier for the user.
Themes make overarching decisions of typography, color and design, this should make it easier for the user to make specific decisions for their website. The customizability that Gutenberg provides must be guided by themes.