How to add fixed navigation to a theme that doesn't have it?

How to add fixed navigation to a theme that doesn't have it?

I recently bought the Purity theme for one of my clients' site. The theme
has a nice minimal look, but its navigation scrolls with the page.
I currently know HTML5 and CSS but not JavaScript or jQuery, and I am
working alone on this (side) project as a volunteer. So I don't want to
spend a lot of time fixing this. Based on this, what is the best way for
me to make the navigation fixed on this theme?
Requirements of this:
Preserve responsiveness and adaptiveness
Fit multiple rows of links into the navigation on narrower displays (we
have 8 top-level navigation items)
I already asked the theme developer, who said it is beyond the scope of
his support because it requires modifying the CSS of the theme.
The site is at http://gospellifebowie.com. It is still in development and
will likely have changed if you're looking at this question much later.