Example theme.json for an existing Classic Theme that excludes many default color and typography controls from being enabled.

theme.json

{
  "schema": "https://schemas.wp.org/trunk/theme.json",
  "version": 2,
  "settings": {
    "layout": {
      "contentSize": "800px",
      "wideSize": "1000px"
    },
    "color": {
      "link": false
    },
    "typography": {
      "customFontSize": false,
      "customLineHeight": false,
      "customFontWeight": false,
      "dropCap": false,
      "fontWeight": false,
      "fontStyle": false,
      "textTransform": false,
      "letterSpacing": false,
      "lineHeight": false,
      "fontSizes": [],
      "fontFamilies": [
        {
          "fontFamily": "Helvetica Neue, Helvetica, Arial, sans-serif",
          "slug": "helvetica-arial"
        }
      ]
    },
    "blocks": {
      "core/button": {
        "spacing": {
          "padding": false
        },
        "color": {
          "defaultPalette": false,
          "customGradient": false,
          "defaultGradients": false,
          "palette": [
            {
              "name": "White",
              "slug": "white",
              "color": "#ffffff"
            },
            {
              "name": "Black",
              "slug": "black",
              "color": "#000000"
            }
          ]
        }
      },
      "core/heading": {
        "spacing": {
          "margin": false
        },
        "color": {
          "text": false,
          "defaultPalette": false
        }
      },
      "core/cover": {
        "spacing": {
          "padding": false
        },
        "color": {
          "text": false,
          "defaultPalette": false,
          "palette": [],
          "custom": false,
          "customGradient": false,
          "defaultGradients": false,
          "gradients": [
            {
              "slug": "vivid-cyan-blue-to-vivid-purple",
              "gradient": "linear-gradient(135deg,rgba(6,147,227,1) 0%,rgb(155,81,224) 100%)",
              "name": "Diagonal cyan blue to vivid purple"
            }
          ]
        }
      }
    }
  }
}