JSFiddle - React, Tailwind, and code Playground

HTML

<script src='https://pocitadlolibise.seznam.cz/pocitadlolibise.js' async defer></script>

<h1>Seamless button</h1>

<p>
<seznam-pocitadlolibise
  entity="https://www.skiclubbrno.cz/?page_id=5932"
  layout="seamless"
  size="large"
  style="width: 128px;"></seznam-pocitadlolibise>
</p>

<h2>Color configuration &amp; preview</h2>

<p>
  <label>
    --primary-color:
    <input name="primary-color" type="color" value="#111111">
  </label>
</p>
<p>
  <label>
    --background-color:
    <input name="background-color" type="color" value="#ffffff">
  </label>
</p>
<p>
  <label>
    --hover-color:
    <input name="hover-color" type="color" value="#888888">
  </label>
</p>
<p>
  <label>
    --count-color:
    <input name="count-color" type="color" value="#888888">
  </label>
</p>
<p>
  <label>
    --active-color:
    <input name="active-color" type="color" value="#de0000">
  </label>
</p>

JavaScript

const button = document.getElementsByTagName('seznam-pocitadlolibise')[0]
const colorNames = [
  'primary-color',
  'background-color',
  'hover-color',
  'count-color',
  'active-color',
]
const colorInputs = colorNames.map(colorName => document.getElementsByName(colorName)[0])
for (const colorInput of colorInputs) {
  colorInput.addEventListener('change', () => {
    button.style.setProperty(`--${colorInput.name}`, colorInput.value)
    const container = button.parentNode
    container.removeChild(button)
    container.appendChild(button)
  })
}