Write Site is a full-service hosting service, and you are welcome to install almost* any plugin or theme.
What do we mean by almost?
For starters, not all plugins and themes are compatible with our server. If you install something that crashes your site or harms other sites on out server, we will disable it. Also, we do not allow sites hosted on our server to hack other sites, host pirated content, or attempt to hack visitors, and if you install anything that falls under that description then we will disable it, and may elect to disable or delete your site.
P.S. If you need help installing a theme or plugin, please contact customer support and we will install it for you.