In Plate there are 2 types of themes. Public themes and Custom themes. Public themes are available when creating a new site, to act as a base for the new site. Custom themes on the other hand are not-reusable themes. You could use a custom theme if you are making a completely custom site, with a custom design, of which you are sure you do not want to (or are not allowed to) reuse.
If you create a theme, it will be a public theme by default. You can change it into a custom theme by linking it to a company. You can do this by selecting an existing company in the theme options in your Partner dashboard.