Change Navigation Bar Colour

Hello,

I want to change the colour of my navigation bar without going through the Colour Customiser.
Could someone please give me the CSS code for this?

Alternatively, could someone tell me why when I turn on the Colour Customiser, the colour of all my “default theme buttons” go from Blue to “transparent”?

My site: new.pentagon.net.au

Thanks for the help.

Kind regards,
Ian.