Add/Modify Header Color Scheme

I love the theme, it has taken me two weeks to find one suitable for my website!..

Now the only change I would like to make to the theme is changing the header and navbar color.

I tried a few plugins i.e Theme tweaker, but those did not work.
Can you provide instructions on how to add/modify the colors or a plugin that works best with your theme to accomplish this task?

Thanks
SG