custom font size presets does not work

I'm also unable to get font sizes to work with the provided instructions. It just shows the default set of font size options.

It should work. But for versions >= 2.6, these settings were deprecated and font sizes should be set at the following section:

- theme-support
  - __experimentalFeatures
  - typography
  - fontSizes
    - theme
      - name: ...

We need to update the documentation.
Anyway, make sure the configuration is at the theme folder and it's called [theme-name].gutenberg.yml. The theme must be active.

I have a custom theme for the front-end but use Claro for admin theme.

In mytheme.gutenberg.yml I tried using your syntax:

- theme-support:
  - __experimentalFeatures:
    - typography:
      - fontSizes:
        - theme:
          - name: "Small"
            slug: "small"
            size: 14
          - name: "Lead"
            slug: "lead"
            size: 20
          - name: "Display 1"
            slug: "display-1"
            size: 96
          - name: "Display 2"
            slug: "display-2"
            size: 88
          - name: "Display 3"
            slug: "display-3"
            size: 72
          - name: "Display 4"
            slug: "display-4"
            size: 56

That didn't seem to work, so I tried without the hyphens:

theme-support:
  __experimentalFeatures:
    typography:
      fontSizes:
        theme:
          - name: "Small"
            slug: "small"
            size: 14
          - name: "Lead"
            slug: "lead"
            size: 20
          - name: "Display 1"
            slug: "display-1"
            size: 96
          - name: "Display 2"
            slug: "display-2"
            size: 88
          - name: "Display 3"
            slug: "display-3"
            size: 72
          - name: "Display 4"
            slug: "display-4"
            size: 56

That worked and I can see a dropdown of my specified font sizes.

I was also able to get custom colors in like this:

theme-support:
  __experimentalFeatures:
    color:
      background: true
      button: true
      caption: true
      defaultDuotone: false
      defaultGradients: false
      text: true
      defaultPalette: false
      heading: true
      link: true
      palette:
        theme:
          - slug: "blue"
            name: "Blue"
            color: "#0000FF"
          - slug: "red"
            name: "Red"
            color: "#CC0000"

Thanks!

Oh.. I put the hyphens for better readability but now I see it just added confusion. Sorry about that. Anyway, I'm glad it work.