Adding your branding to your site is really simple. Go to the setup icon and under site settings select settings. Now select the branding tab.
First select your primary colour. This will be applied to the header bar and some of the buttons. You can use the colour palette to select your colour and shade, or if you know the hex number, type that into the box below. Once you are happy with your colour click apply.
Now select your secondary colour in the same way. Once you are happy with both colours, click apply.