You change those. In your WordPress dashboard, go to the Appearance section and select Theme Options. Then click on the Navigation tab at the top. You’ll find font and font size options for the navigation menu at the top. The options to change the various colors in the navigation menu will be below those.
If you run into any trouble making those changes, let me know.
Replace the rgba(0, 0, 0, 0.81) with the color you’d like to use. You can use a hex color like #000000 for black or an RGBA color if you need it semi-transparent. There is a RGBA color picker at http://www.css3maker.com/css-3-rgba.html you can use.