Fix: Apply purgecss and safe the darkmode from it

[Marty]

I don't know how this ever worked.
The purgecss plugin was not applied. Maybe it was only to show how it could be done.
But maybe the bug was known, that it breaks the darkmode.

Nonetheless, it works now and brought my css from 7mb to 18kb :)

[apvarun]

@Marty This change is not required. Moreover, @fullhuman/postcss-purgecss can be removed completely since tailwindcss itself takes care of the purging when the HUGO_ENVIRONMENT is production

Executing the below command directly in the repo will provide you with purged css

NODE_ENV=production hugo -s exampleSite --themesDir=../.. --gc

[Marty]

Yes, thanks for pointing that out. I'm just getting started with hugo, tailwind etc. :)