From Newired Portal, you can enable and disable all available themes. In order to do that, go to the Addons page of the Portal.
From here, scroll down until you get to the themes section.
Each theme can be enabled/disabled by clicking on the corresponding toggle on the left. In the image above, all themes are currently enabled.
If you disable a theme that is currently in use by one or more of your sites, it will be automatically replaced by the built-in default (green) theme.
If you re-enable that theme, it will be automatically selected as the used theme again.