To uninstall: first select Walnut as the current theme in the Theme/Addons manager, restart firefox, then uninstall and then restart Firefox.
Please don't use the review box for any bug reports. Contact me through the email address provided under the 'Support' heading. Using email is the quickest way to notify me, and generally I will try to make a fix available as quickly as possible.
Note, as the AMO approval process takes ages, it may be that there is a newer version available as 'experimental' than the one currently displayed. Go to 'View All Versions' (at bottom of this page), and check if there is a newer (experimental) version available there.