Main menu colors are broken

The sample Blue Planet theme shows the main menu options as white or very light gray text with blue backgrounds. On my website, the unselected menu options are light gray text on white backgrounds; unless the user is viewing the screen from the correct angle, the text is not even visible. I need to either make the text darker so it’s visible, or I need the background color of the unselected tabs to be darker so there’s enough contrast for the user to see the unselected main menu tabs and so that the text easily stands out.