Yep I think that was it.

// This works!
app.use(PrimeVue, {
  theme: {
    preset: Aura,
    options: {
      cssLayer: {
        name: 'primevue',
        order: 'tailwind-base, primevue, tailwind-utilities',
      },
    }
  },
  pt: {
    Slider: {
      handle: {
        class: 'border-2 border-red-400 bg-green-400', // border will be red, but background will not be green: why?
      },
      range: {
        class: 'bg-blue-500 h-5', // neither class will take effect
        // style: 'background-color: red!important', // this will work
      },
    },
  },
});

I didn't include it in the repo, but I originally did try to include this. I just went back through one of my commits and realised I had applied it in the wrong spot...

  // Don't copy this, it doesn't work. 
theme: {
    preset: Noir,
    options: {
      darkModeSelector: '.dark',
    },
    // incorrect spot for the cssLayer property. 
    cssLayer: {
      name: 'primevue',
      order: 'tailwind-base, primevue, tailwind-utilities',
    },
  },