By default, Skype doesn't play very nicely if you use a theme with dark menus. This is a serious inconvenient since Skype is a very popular application and the default Ubuntu theme - Ambiance - uses dark menus. But this is actually very easy to "fix".
To make the Skype menu readable when using a dark theme, go to the Skype options (you need to be logged in to do this) and on the "General" tab, under "Choose style" select "Desktop settings":
Then, click "Apply" and restart Skype. The Skype menus should now be readable.