This plugin does not protect itself from the themes that may be installed and other modules - the CSS is complex, though does not contain enough context selectors to prevent it from breaking with some themes. It kind of makes too many assumptions.
However, if it works for you, it works really well and is nicely put together. It is a shame it is not compatible with the theme I am using, and not easily fixed.
Update: the author of this plugin has been very reactive and helpful in fixing the CSS in the plugin to work even better with themes. I'll leave my original comments above for context, but can state within days this was all fixed. Good work :-)