STRIVE

PostCSS Magento 2 system., _(*1)

Requirements

Magento 2 CE installed
NodeJS (with npm package manager)
GulpJS $ npm install -g gulp (sudo may be needed for mac users)

Installation

Install via Composer: composer require trive/theme-frontend-strive or clone this repository under Magento /app/design/frontend/Trive/blank folder & StriveGulpTool inside Magento_root/strive-gulp folder.
bin/magento setup:upgrade
bin/magento setup:static-content:deploy
Install node_modules with npm install or yarn inside strive-gulp folder
Set up Gulp file paths inside gulpfile.

Features & Usage

Fast css compilation thanks to PostCSS
PostCSS theme fallback / child theme support
Small (as possible) set of PostCSS base styles and settings needed to get a working development system. Hey, it looks somehow like a Magento base theme, right? ;)
BrowserSync - instant css injecting into browser & weinre support
Theme static content is watched & transferred directly into pub/static via Gulp. No need to use Magento content deploy during development.
Added gulp clean and deploy tasks. If there is a need, gulp clean will remove everything in var/cache, var/generation, var/view_preprocessed & pub/static folder (Except .htaccess, magento blank & magento luma theme) and gulp deploy will deploy Magento static content (short of bin/magento setup:static-content:deploy)., _(*2)
PostCSS plugins used on Strive (check npmjs for usage):, _(*3)
postcss-import - PostCSS plugin to transform @import rules by inlining content.
postcss-simple-vars - PostCSS plugin for Sass-like variables
postcss-extend - As close to cssnext @extend as possible for PostCSS
postcss-nested - PostCSS plugin to unwrap nested rules like how Sass does it.
autoprefixer - Parse CSS and add vendor prefixes to CSS rules using values from the Can I Use website
cssnano - A modular minifier, built on top of the PostCSS ecosystem.
css-mqpacker - Pack same CSS media query rules into one media query rule.
postcss-mixins - PostCSS plugin for mixins
lost - LostGrid is a powerful grid system built in PostCSS that works with any preprocessor and even vanilla CSS.
postcss-responsive-type - PostCSS plugin that adds responsive magic to font-size
postcss-custom-media - PostCSS plugin to transform W3C CSS Custom Media Queries to more compatible CSS
postcss-color-function - PostCSS plugin to transform W3C CSS color function to more compatible CSS.

Gulp

Creating a child theme

Gulpfile paths.cssSrc would be 'app/design/fronted/Trive/theme-name' and paths.cssDest: 'pub/static/frontend/Trive/theme-name/'. Child theme will need to have styles.css and settings.css inside the same folder structure like Strive (web/src/preCSS/), so it'll be needed to copy those two files from Trive/blank to child theme., _(*4)

Override css files

'postcss-import' looks for imported css files inside Trive/theme-name and if it can't find them here, it'll look into Trive/blank. To override existing blank theme css file, add css file with same name and path in child theme. postcss-import will take it instead of Trive/blank css., _(*5)

File override example: Trive/blank/web/src/preCSS/theme/icons.css -> Trive/theme-name/web/src/preCSS/theme/icons.css, _(*6)

Gulp Tasks

gulp - Default gulp task (runs 'css', 'static', 'watch' & 'browser-sync' tasks). It watches css, html, images & js files. Browser-sync injects css file into the browser. These files, if changed, are auto transferred from theme to pub/static folder, so deployment of Magento static content would not be needed. Gulp-watch package is added to gulpfile so newly added files are watched also. This means it's not needed to restart gulp task anymore., _(*7)
gulp clean - Removes everything in var/cache, var/generation, var/view_preprocessed & pub/static folder (Except .htaccess, magento blank & magento luma theme), _(*8)
gulp deploy - It's a short of bin/magento setup:static-content:deploy. There is no problem to use longer option either :), _(*9)

Update

gulp upgrade - Short of 'bin/magento setup:upgrade', _(*10)
gulp deploy - Excluded Magento/blank & Mageto/luma by default. Faster deployment this way..., _(*11)