How to set default layout for specific folder?

[DrSensor]

Is it possible to set the default layout for specific folder of html files?
For example, I want to set the default layout of demo/**/index.html to _includes/demo.njk

.
├── LICENSE
├── _config.ts
├── core
├── demo ------------ layout: _includes/demo.njk
│   ├── _data.yaml
│   ├── console
│   │   ├── index.html
│   │   ├── module.js
│   │   └── test.ts
│   ├── counter
│   │   ├── index.html
│   │   ├── module.js
│   │   └── test.ts
│   ├── multi-counter
│   │   ├── index.html
│   │   ├── module.js
│   │   └── test.ts
│   ├── page.test.ts
│   ├── style.css
│   └── tester.ts
├── deno.json
├── deno.lock
├── import_map.json
└── package.json

Note: I'm using index.html instead of index.njk

[oscarotero]

Just add layout: demo.njk to the demo/_data.yaml file, so this value is shared by all pages in this folder.