New opacity shortcut not working with custom colors

[lilnoes]

What version of Tailwind CSS are you using?

v2.2.2

What build tool (or framework if it abstracts the build tool) are you using?

tailwindcss cli

What version of Node.js are you using?

v16.3.0

What browser are you using?

Chrome

What operating system are you using?

Ubuntu 20

Reproduction repository

https://github.com/lilnoes/tailwindbug

Describe your issue

tailwindcss v2.2 introduced a new way to add opacity to color utilities like
bg-white/20, but "bg-[wheat]/20" is not working, background color is not changing. I expected "bg-[wheat]/20" to work like "bg-[wheat] bg-opacity-20"

[adamwathan]

Hey, should be fixed here!

#4723

Can test it out using npm install tailwindcss@insiders, fix will land in v3.