PrestaShop1.7.x: registering custom assets in child theme for all, not product

Question

In PrestaShop (PS) 1.7, I'd like to register an additional css asset as explained in the documentation:

By now you probably understood that this theme.yml file became the heart of PrestaShop themes.

To register assets, create a new assets key at the top level of your theme.yml, and register your files according to your needs. Page identifiers are based on the php_self property of each controller (example)

For example, if you want to add an external library on each page and a custom library on the Product page:

assets:
  css:
    product:
      - id: product-extra-style
        path: assets/css/product.css
        media: all
        priority: 100

But, instead of doing so for a product, I want to do it for all:

 assets:
   use_parent_assets: true
   css:
     all:
       - id: my-custom-colors.css
         path: assets/css/my-custom-colors.css
         priority: 51

This appears not to be working. It is also extremely similar to this question on the PS forum, which is as of today unanswered.

Can this be done at all, or am I originally at fault by not understanding the docs?