Unable to register a css file in my theme

[prak]

Hello, I'm trying to make a new theme for prestashop 1.7, I want to include a custom css file.

I have followed instructions in the asset management page( http://developers.prestashop.com/themes/assets/index.html) and tried to register a custom css file with this code in the theme.yml, below the 'author' key and the above the 'meta' key

this way:

assets:
  css:
    all:
      -id: mytheme-css
      path: assets/css/mytheme.css
      media: all
      priority: 100

The css file isn't loaded. I am a beginner to yml.

Help would be greatly appreciated!