Google Apps Script that adds a "Format Code" menu option for documents.

FormatCode.gs

// Source: https://webapps.stackexchange.com/a/117682
// Docs: https://developers.google.com/apps-script/reference/document/

// Add new menu item
function onOpen() {
  DocumentApp.getUi()
  .createMenu('Styles')
  .addItem('Format Code', 'formatCode')
  .addToUi();
}

// Define code styling
var fontFamily = "Roboto Mono";
var style = {};
style[DocumentApp.Attribute.FONT_SIZE] = 9;
style[DocumentApp.Attribute.BACKGROUND_COLOR] = "#E7E7E7";
style[DocumentApp.Attribute.FOREGROUND_COLOR] = "#333333";
style[DocumentApp.Attribute.BOLD] = false;

// Apply code formatting
function formatCode() {
  var selection = DocumentApp.getActiveDocument().getSelection();
  if (selection) {
    var elements = selection.getRangeElements();
    for (var i = 0; i < elements.length; i++) {
      var element = elements[i];

      // Only modify elements that can be edited as text; skip images and other non-text elements.
      if (element.getElement().editAsText) {
        var text = element.getElement().editAsText();

        // Style the selected part of the element, or the full element if it's completely selected.
        if (element.isPartial()) {
          text.setFontFamily(element.getStartOffset(), element.getEndOffsetInclusive(), fontFamily);
          text.setAttributes(element.getStartOffset(), element.getEndOffsetInclusive(), style);
        } else {
          text.setFontFamily(fontFamily);
          text.setAttributes(style);
        }
      }
    }
  }
}