From https://stackoverflow.com/questions/23820703/how-to-inject-content-of-css-file-into-html-in-gulp

gulp-replace-file-link-with-file-content.js

var gulp = require('gulp'),
    replace = require('gulp-replace'),
    fs = require('fs');

// in your task
return gulp.src(...)
  .pipe(replace(/<link href="style.css"[^>]*>/, function(s) {
      var style = fs.readFileSync('style.css', 'utf8');
      return '<style>\n' + style + '\n</style>';
  }))
  .pipe(gulp.dest(...));
  
// or for multi files
  return gulp.src(...)
  .pipe(replace(/<link href="([^\.]+\.css)"[^>]*>/g, function(s, filename) {
      var style = fs.readFileSync(filename, 'utf8');
      return '<style>\n' + style + '\n</style>';
  }))
  .pipe(gulp.dest(...));