Transparent background on wysiwyg highlight mode

Steps to reproduce:

1. Activate Flamingo skin and color theme for 6.1rc1 (do not know if this is mandatory)
2. Go to Administration > wysiwyg and add the color plugin and backcolor toolbar
3. Edit a page with wysiwyg and try to highlight some text

Result: http://screencast.com/t/t1aep6Wld