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const customType = (editor) => {
  editor.StyleManager.addType('my-custom-prop', {
    // Create UI
    create({ props, change }) {
      const el = document.createElement('div');
      el.innerHTML = `<input type="range" class="my-input" min="${props.min}" max="${props.max}"/>`;
      const inputEl = el.querySelector('.my-input');
      inputEl.addEventListener('change', event => change({ event })); // `change` will trigger the emit
      inputEl.addEventListener('input', event => change({ event, partial: true }));
      return el;
    },
    // Propagate UI changes up to the targets
    emit({ props, updateStyle }, { event, partial }) {
      const { value } = event.target;
      updateStyle(`${value}px`, { partial });
    },
    // Update UI (eg. when the target is changed)
    update({ value, el }) {
      el.querySelector('.my-input').value = parseInt(value, 10);
    },
    // Clean the memory from side effects if necessary (eg. global event listeners, etc.)
    destroy() {
    },
  });
};

const editor = grapesjs.init({
  container: '#gjs',
  fromElement: true,
  height: '100%',
  storageManager: false,
  noticeOnUnload: false,
  plugins: [customType],
  styleManager: {
    sectors: [
      {
        name: 'My sector',
        properties: [
          {
            type: 'my-custom-prop',
            property: 'font-size',
            default: '15',
            min: 10,
            max: 70,
          },
        ],
      },
    ],
  },
});

<div id="gjs">
  <div style="padding: 25px">Custom Style Type</div>
  <div style="font-size: 50px">Value 50</div>
</div>

body, html {
  margin: 0;
  height: 100%;
}

.selectors-wrp {
  padding: 10px 5px;
}
.selectors-head {
  display: flex;
  align-items: center;
}
.selectors-title {
  flex-grow: 1;
  text-align: left;
}
.selectors {
  display: flex;
  flex-wrap: wrap;
  background-color: rgb(0 0 0 / 15%);
  padding: 3px 5px;
  border-radius: 3px;
  margin: 10px 0;
  gap: 5px;
}
.selector, .selector-add {
  display: flex;
  gap: 3px;
  background-color: #8f4a79;
  color: white;
  padding: 2px 5px 3px;
  border-radius: 3px;
  align-items: center;
  white-space: nowrap;
}
.selector-add {
  font-size: 1rem;
  padding: 2px 8px 3px;
  background-color: rgb(255 255 255 / 25%);
  cursor: pointer;
}
.selector-rm {
  font-size: 1rem;
  cursor: pointer;
}
.selectors-targets {
  text-align: left;
}