background color doesn't work with breakpoint keywords

Here a simple example:

```html
<div class="h-20rem w-20rem bg-blue-200 sm:bg-orange-200 md:bg-green-200 lg:bg-red-200 xl:bg-yellow-200"></div>
```

If I understood well, the square should change its color when we resize the window. But nothing happens.

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Original issue:
https://github.com/primefaces/primeflex/issues/130
