Symptoms

Users experience that drop-down menu background are transparent when using Confluence 4.3.x. This does not cause any functional issues, only a display problem, however in some cases (especially with dark background) could make those menus practically unusable.

Cause

This problem appears when a third-party plugin is installed: SubSpace Plugin

Workaround

As a workaround suggested by Judith in her comment, if you enable the Confluence 4 Theme Integration module of the SubSpace Plugin, the issue disappears. This module is disabled by default in version 1.7.4.