Try using a browser developer tool for this kind of exploratory CSS work.
I used Google Chrome's built-in Developer Tool and managed to get your menu transparent (right of toolbar in screenshot) [screenshot].
Now that you know the style, from being exposed/added by the Developer Tool, you can add a new style to override it.
You ought to add new styles externally, not within the theme's files. Changes to the theme's files will compromise the theme and be compromised when the theme updates.
External changes can be made using a Child Theme or through a plugin that provides Custom CSS functionality.
I recommend using the Jetpack plugin and its Custom CSS functionality to hold your CSS modifications.