Well, given that the query is about the header navbar of the Academi theme from the plugins database) and that reply is a link to info about changing css for the entire colour scheme of a commercial theme by a different author, and that a check through the Academi theme using a 'find in files' tool in an editor shows that quite a lot of that css is meaningless in Academi because the selectors are just not used in the theme you are trying to customise - I'd probably say don't use any of it. It's also actually incorrect in the specific of what you asked about, even if you just took the few lines in there that do apply to the heading bar of that particular theme - they wont apply in Academi because it uses different selectors!
In Boost (which Academi has as a parent theme) it would be
background-color: #ffff00; // Change your colour here
Although in Academi it may simply be .navbar rather than .navbar-light - the best way to check is to use a tool such as firebug or Chrome developer tools to identify the correct selector being used.
EDIT: Just installed Academi and it actually overrides the navar-light colour with #header, so you need to add the code below to the Academi CustomCSS setting.
border-bottom: 1px solid #e8e3e3;
background: red; // Your colour here
(I've left the border colour reference in the example in case you also need/want to change that)