(From official document) Bundling Custom css with theme’s assets

For adding custom css to be bundled inside one minimized css: Create folder & file in your project directory as

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.(site root)
├── config.yml
├── content/
├── theme/hugo-PaperMod/
└── assets/
    └── css/
        └── extended/  <---
            ├── custom_css1.css <---
            └── any_name.css   <---

(All css files inside assets/css/extended will be bundled !)

Note: blank.css is just the placeholder so that it doesn’t break the theme when no files are present under assets/css/extended

Reference