ThemeOverride.js

/*
Idea stolen from https://gist.github.com/thomasloven/2a37152725c582fec4420ecedb65ebe3
Add this to your configuration.yaml
frontend:
  extra_module_url:
    - /local/ThemeOverride.js
And put the following into <config-dir>/www/ThemeOverride.js
*/

document.documentElement.style.setProperty('--ha-card-border-radius', '20px');

window.addEventListener("hass-more-info", () => { window.setTimeout(styleMoreInfo, 1)});
function styleMoreInfo() {
  var dialog = document.querySelector("home-assistant")._moreInfoEl
  dialog.style.borderRadius = "20px";

  var dialogHeader = dialog && dialog.shadowRoot;
  dialogHeader = dialogHeader && dialogHeader.querySelector("more-info-controls");
  dialogHeader = dialogHeader && dialogHeader.shadowRoot;
  dialogHeader = dialogHeader && dialogHeader.querySelector("app-toolbar");
  if (dialogHeader !== null) {
    dialogHeader.style.borderRadius = "20px 20px 0 0";
  }
}