Question: I am using fontspec and beamer and would like to change the bullet symbol of "itemize" to a given character. 

I have tried 

```
\setbeamertemplate{itemize item}[]

```

Before and after `\begin{document}`, but it did not work.

So far, the only thing that works is `\item[☙]`, but it's too cumbersome to use every time I want an item.

A MWE looks like this:

```
\documentclass[xcolor=dvipsnames]{beamer} 
\setbeamertemplate{itemize item}[☙]
\usepackage{fontspec}
\setmainfont{EB Garamond}
\usefonttheme{serif}

\begin{document}

\begin{frame}
\begin{itemize}
\item \item \item
\end{itemize}
\end{frame}

\end{document}

```

Answer: 
The `[☙]` option is not predefined, so it has no effect; the syntax should be like

```
\setbeamertemplate{itemize item}[circle]

```

where `circle` has been defined with `\defbeamertemplate`. Here's a complete example

```
\documentclass[xcolor=dvipsnames]{beamer}
\usepackage{fontspec}
\setmainfont{EB Garamond}
\usefonttheme{serif}

\defbeamertemplate{itemize item}{fleuron}{☙}
\setbeamertemplate{itemize item}[fleuron]

\begin{document}

\begin{frame}
\begin{itemize}
\item
\item
\item
\end{itemize}
\end{frame}

\end{document}

```

![enter image description here](https://i.stack.imgur.com/k2qsW.png)
It's not necessary "define" a style, though it's the best solution. You also can write

```
\setbeamertemplate{itemize item}{☙}

```

(with braces, not square brackets).