A lot of the solutions don't work inline. This is a cleaned-up solution offered by @AlekseyBykov using Google App Scripts to add a custom menu action:

1. Create a new script (Extensions > Apps Script)
2. Copy the following code into the editor:

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

// Define code styling
var style = {};
style[DocumentApp.Attribute.FONT_FAMILY] = DocumentApp.FontFamily.CONSOLAS;
style[DocumentApp.Attribute.FONT_SIZE] = 10;
style[DocumentApp.Attribute.BACKGROUND_COLOR] = "#DDDDDD";
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.setAttributes(element.getStartOffset(), element.getEndOffsetInclusive(), style);
        } else {
          text.setAttributes(style);
        }
      }
    }
  }
}

3. Choose Deploy > "New deployment"
4. Use the new menu item to format selected text (Styles > Format Code)

A lot of the solutions don't work inline. This is a cleaned-up solution offered by @AlekseyBykov using Google App Scripts to add a custom menu action:

1. Create a new script Extensions > Apps Script
2. Copy the following code into the editor:

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

// Define code styling
var style = {};
style[DocumentApp.Attribute.FONT_FAMILY] = DocumentApp.FontFamily.CONSOLAS;
style[DocumentApp.Attribute.FONT_SIZE] = 10;
style[DocumentApp.Attribute.BACKGROUND_COLOR] = "#DDDDDD";
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.setAttributes(element.getStartOffset(), element.getEndOffsetInclusive(), style);
        } else {
          text.setAttributes(style);
        }
      }
    }
  }
}

3. Run the onOpen function once from the code editor to trigger the authorization prompts and approve them.
4. Use the new menu in your document to format selected text Styles > Format Code

A lot of the solutions don't work inline. This is a cleaned-up solution offered by @AlekseyBykov using Google App Scripts to add a custom menu action:

1. Create a new script (Tools > Script Editor)
2. Copy the following code into the editor:

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

// Define code styling
var style = {};
style[DocumentApp.Attribute.FONT_FAMILY] = DocumentApp.FontFamily.CONSOLAS;
style[DocumentApp.Attribute.FONT_SIZE] = 10;
style[DocumentApp.Attribute.BACKGROUND_COLOR] = "#DDDDDD";
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.setAttributes(element.getStartOffset(), element.getEndOffsetInclusive(), style);
        } else {
          text.setAttributes(style);
        }
      }
    }
  }
}

3. Assign a trigger to run the onOpen function "On open" (Edit > Current Project's Triggers)
4. After authorizing the script, reload the original document
5. Use the new menu item to format selected text (Styles > Format Code)

A lot of the solutions don't work inline. This is a cleaned-up solution offered by @AlekseyBykov using Google App Scripts to add a custom menu action:

1. Create a new script (Extensions > Apps Script)
2. Copy the following code into the editor:

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

// Define code styling
var style = {};
style[DocumentApp.Attribute.FONT_FAMILY] = DocumentApp.FontFamily.CONSOLAS;
style[DocumentApp.Attribute.FONT_SIZE] = 10;
style[DocumentApp.Attribute.BACKGROUND_COLOR] = "#DDDDDD";
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.setAttributes(element.getStartOffset(), element.getEndOffsetInclusive(), style);
        } else {
          text.setAttributes(style);
        }
      }
    }
  }
}

3. Choose Deploy > "New deployment"
4. Use the new menu item to format selected text (Styles > Format Code)