Someone has added a fixed color css in Confluence 5.5.6

/* module-key = 'com.atlassian.confluence.plugins.expand-macro:expand-macro-css-only', location = 'com/atlassian/confluence/plugins/expand/css/expand-macro.css' */
@media all {
...
.expand-control-text

...
decoration:none; }
}

I fixed it with some custom css, but in general Confluence should become better in respecting its own color settings.