After much research, I found a solution to the problem (it was much simpler than I thought)
First, to configure package.json, I ran the following commands in the terminal, inside the project folder, to install the necessary resources:
npm init
npm install postcss-cli autoprefixer
npm i -D @fullhuman/postcss-purgecss
I put the post.config.js in the root folder of the project (it was in the theme folder before), with the following settings:
module.exports = {
plugins: {
'@fullhuman/postcss-purgecss': {
content: ['./**/*.html', './**/*.js'],
},
autoprefixer: {
browsers: [
"last 2 versions",
"Explorer >= 8",
]
},
}
};
And the head.html layout inside the theme folder was also much cleaner:
{{ $styles := resources.Get "scss/main.scss" | toCSS | postCSS | minify }}
<style>
{{- $styles.Content | safeCSS -}}
</style>
And the result of that was:
From 1021 rules of CSS to 165 rules
thanks for replies @elperronegro @h-enk