See eg for themes:

Workflow to install a new theme is too go to "Marketplace => Themes" and click on a theme to install and activate it. Now the theme is active and the user can give it a try. If a user doesn't want to use the theme anymore it's very hard to find how to go back to the initial theme. Someone has to find out that it is a plugin and that they can be managed under "Plugins". The theme does not appear in the plugins list so one has to find the little "Manage Themes" link in the introduction of the plugins page. As soon as someone is on the "Manage Themes" page one has to find out that one has to deactivate the current theme to activate the initial theme see #9569

It would be nice to instead of showing "Installed" in the Marketplace for an already installed plugin to show the option to uninstall or at least to deactivate it. This way a user can basically undo it's action where it was done initially and this should be much more intuitive.

This also applies for plugins and not only for themes. It's not trivial once a new plugin is installed to find out where to uninstall it. Plugin etc is rather technical and for normal users this might be not really usable.

Of course only a super user would have this option.