card-modder.js

import {html, LitElement} from "https://unpkg.com/@polymer/lit-element?module";
class CardModder extends LitElement {

  static get properties() {
    return {
      hass: Object,
      config: Object,
    };
  }

  render()
  {
    return html`${this.card}`;
  }

  async setConfig(config) {
    this.config = config;

    let tag = this.config.card.type;
    if(tag.startsWith("custom:"))
      tag = tag.substr(7);
    else
      tag = `hui-${tag}-card`;
    this.card = document.createElement(tag);
    this.card.setConfig(config.card);

    if(this.card.updateComplete) {
      await this.card.updateComplete;
    }
    this._cardMod();
  }

  async _cardMod() {
    let target = this.card;

    let maxDelay = 5000;
    while(maxDelay) {
      if(this.card.shadowRoot &&
          this.card.shadowRoot.querySelector("ha-card")) {
        target = this.card.shadowRoot.querySelector("ha-card");
        break;
      } else if(this.card.querySelector("ha-card")) {
        target = this.card.querySelector("ha-card");
        break;
      } else if(this.card.firstChild && this.card.firstChild.shadowRoot &&
          this.card.firstChild.shadowRoot.querySelector("ha-card")) {
        target = this.card.firstChild.shadowRoot.querySelector("ha-card");
        break;
      }

      maxDelay -= 100;
      await new Promise(resolve => setTimeout( () => resolve() , 100));
    }

    for(var k in this.config.style) {
      target.style.setProperty(k, this.config.style[k]);
    }

  }

  set hass(hass) {
    if(this.card) this.card.hass = hass;
  }

  getCardSize() {
    return this.card.getCardSize();
  }
}

customElements.define('card-modder', CardModder);