How to use an external css-file in conjunction with the default marp theme?

Hm, I’m feeling it’s weird. I’ve tried to use default theme + custom styling in minimum assets and it is working correctly.

---
theme: custom-theme
---

# Hello, world!

/* custom-theme.css */
/* @theme custom-theme */

@import 'default';

section {
  /* Override default background */
  background: #def;
}

marp --engine ./engine.js --bespoke.progress --watch --theme-set custom-theme.css -- mymarkdownfile.md

@yhatt, I require customization in order to use extensions (markdown-it-mark, markdown-it-container, markdown-it-attrs etc.); therefore, I am using the engine.js implementation as stated in the docs.

Thanks for the code fragment; my code in engine.js looks identical to yours

// engine.js
const { Marp } = require('@marp-team/marp-core')

const markdownItMark = require('markdown-it-mark')
const markdownItAttrs = require('markdown-it-attrs')
const markdownItContainer = require('markdown-it-container')
const markdownItFootnote = require('markdown-it-footnote')
const markdownItDiv = require('markdown-it-div')
// const markdownIt = require('markdown-it')

module.exports = (opts) => new Marp(opts)
// .use(markdownIt)
.use(markdownItMark)
.use(markdownItAttrs)
.use(markdownItContainer, 'definition')
.use(markdownItContainer, 'task')
.use(markdownItContainer, 'note')
.use(markdownItContainer, 'columns')
.use(markdownItContainer, '1st-col')
.use(markdownItContainer, '2nd-col')
.use(markdownItContainer, '3nd-col')
// .use(markdownItFootnote) // disabled because it causes error with marp-cli
.use(markdownItDiv)
.use(require('markdown-it-fontawesome'));

I used the marp class exactly as you described but then I could not make the marp-cli to use both, the default theme and my custom theme in conjunction.

Anyway, not be best solution but I could still revert to including all style information in the markdown-file, although I would love to have them separated to reuse the custom-theme in other marp projects.