Invalid XHTML on theme selector page when custom_corners selected: $THEME unset

When custom_corners theme is used, viewing the theme selection page produces XHTML warnings about 4 </div> tags missing. These refer to the closing divs for the custom corners theme. The reason is that, at the start of each iteration of the [foreach $theme] loop, the global $THEME is unset to allow for proper displaying of each theme thumbnail preview. At the end of the loop, the global $THEME is set to the last theme in the loop, instead of the original custom_corners.