The fact that you need less classes in Miligram (the properties of the HTML tags are defined in the CSS) makes no difference! The question of the OP was:
Can i just apply css to existing html file?
I am new to Pinegrow and I am not an expert in CSS. I only google for CSS solutions whenever I want to adjust my website/CMS.
My question is, can I just apply Bootstrap or Tailwind CSS to this file to make it looks better?
And my answer was that it’s better to build a excising site from the bottom and transfer the content using Bootstrap, because all you need for that is available in Pinegrow. Miligram is not going to do anything without knowledge of HTML and CSS or you get (at best) a uniform look like all the Bootstrap websites that look the same, because most users don’t understand how to style their own stuff. But looking at the Miligram CSS I doubt that anything sensible comes out if you add it to a existing website!
Bootstrap comes with tons of components that you can add to your pages and style in Pinegrow.