That's because fonts is a collection of the singular font. Colors, the same for color. fontFamily is precisely what is says; a font family. It's a singular unit. There is no inconsistency here.
When configuring the settings, it uses `fontFamily`, singular, to set up multiple font family options (eg. options for "heading" and "body" in their example). `colors`, however, is plural here.
When applying them later in the code, they're both singular since they're specifying a specific family/color.
I agree with GP that this inconsistency shakes my confidence in the package.
Tailwind is free and open source. If it really shakes your confidence with the entire system, you could certainly consider opening a discussion or PR. I've been using Tailwind in production and I never even really thought about it. It just seems out of place that of all the incredible work Tailwind is doing, this potentially minor issue is enough to say "well then nevermind".
When applying them later in the code, they're both singular since they're specifying a specific family/color.
I agree with GP that this inconsistency shakes my confidence in the package.