(theme: Hugo-classic)
To use a custom domain we must create a file static/CNAME that only contains the CNAME of the custom domain (which, in this case, is www.johannesnauta.com). See: https://gohugo.io/hosting-and-deployment/hosting-on-github/
Then, just push to the main branch, and Github will set in motion what needs to happen. The details on the Github Action are in .github/workflows/gh-pages.yml.
Most content is in the /content/ folder. Images, or other static items, can be put into /static/. Please find other examples, such as _index.md how to add an image.
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The theme contains css in themes/hugo-classic/