findKeywords.gs

function onOpen() {
  const ui = DocumentApp.getUi();
  ui.createMenu('SEO')
      .addItem('Check Keywords', 'findKeywords')
      .addToUi();
}

function makeCaseInsensitive(searchPattern) {
  // see: https://stackoverflow.com/a/54217083/11582326
  return "(?i)" + searchPattern;
}

function highlight(text, matchedRange) {
  text.setBackgroundColor(matchedRange.getStartOffset(), 
                          matchedRange.getEndOffsetInclusive(),
                          "#FFA500");
}

function findKeywordsInParagraph(keyword) {
  return function (paragraph) {
    // get paragraph's text that we can modify
    const Text = paragraph.editAsText();
    
    // turn the given keyword into a search pattern by making it
    // case insensitive
    const SearchPattern = makeCaseInsensitive(keyword);
    
    // find matching pieces of text
    var Range = paragraph.findText(SearchPattern);
    while (Range) {
      // highlight found match...
      highlight(Text, Range);
      // ...and try finding the next one
      Range = paragraph.findText(SearchPattern, Range);
    }
  }
}

function askUser() {
  const ui = DocumentApp.getUi();
  const response = ui.prompt('Enter your keyword:');

  // Process the user's response.
  if (response.getSelectedButton() == ui.Button.OK) {
    return response.getResponseText();
  } else {
    // user decided not to give us their keywords :(
    return null;
  }
}

function findKeywords() {
  const activeDoc = DocumentApp.getActiveDocument();
  const paragraphs = activeDoc.getBody().getParagraphs();

  // fire up a UI form asking the user for a keyword
  const keyword = askUser();
  
  // proceed only if user had provided us with a keyword to fetch
  if (keyword) {
    // go through the list of paragraphs and highlight
    // every encountered keyword in them
    paragraphs.forEach(findKeywordsInParagraph(keyword));
  }
}