Add info about the high contrast theme

doc/metropolistheme.dtx

@@ -436,6 +436,13 @@ of \themename specific colors, which can also be redefined to your liking.
 \setbeamercolor{progress bar in section page}{ ... }
 \end{lstlisting}
 
+For low-light situations \themename it might be helpful to use the
+|metropolis-highcontrast| color theme. It is enabled like any other color theme:
+
+\begin{lstlisting}
+\usecolortheme{metropolis-highcontrast}
+\end{lstlisting}
+
 
 \subsection{Font Customization}