changedetection.io/changedetectionio/static/styles/scss/parts/_darkmode.scss

#toggle-light-mode {
  width: 3rem;
  /* default */
  .icon-dark {
    display: none;
  }

}

html[data-darkmode="true"] {
  #toggle-light-mode {
    .icon-light {
      display: none;
    }

    .icon-dark {
      display: block;
    }
  }
}