Highlighting in shadow DOM

The option shadowDOM: true allows to highlight a text inside shadow DOMs that have mode: 'open' option and are already created.
You can play with Playground - Examples -> Shadow DOM.

To customize the style of mark elements in shadow DOM, the option shadowDOM: {style: 'your mark element style'} can be used.
It creates a style element and appends it at the end of shadow root child nodes (before v2.6.0 it inserted the style at the beginning of shadow root child nodes).

An inline style can be used as an alternative:

each: (markElement, info) => {
  // a shadow root is the DocumentFragment
  if (markElement.getRootNode().nodeType === Node.DOCUMENT_FRAGMENT_NODE) {
    markElement.style.color = "red";
  }
}