代码突出显示在rstudio ioslides演示文稿中不起作用？

Question

我不明白如何在要通过Rmd文件构建的ioslides演示文稿中打开代码突出显示：

---
title: "test"
author: "Florian Oswald"
date: "05/10/2018"
output: 
  ioslides_presentation:
    highlight: textmate
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = FALSE)
```

## R Markdown

This is an R Markdown presentation. Markdown is a simple formatting syntax for authoring HTML, PDF, and MS Word documents. For more details on using R Markdown see <http://rmarkdown.rstudio.com>.

When you click the **Knit** button a document will be generated that includes both content as well as the output of any embedded R code chunks within the document.

## Slide with Bullets

- Bullet 1
- Bullet 2
- Bullet 3

## Slide with R Output

```{r cars, echo = TRUE}
summary(cars)
lm(formula = speed ~ dist, data =cars)
```

## Slide with Plot

```{r pressure}
plot(pressure)
```

产生