Add default empty argument to \plain

README.md

@@ -17,11 +17,18 @@ Thus to compile the demo with `make` you must
 * install the [Fira Sans](https://github.com/mozilla/Fira) font and
 * have a usable TikZ installation.
 
+
 ### Package options
 
 * `titleprogressbar` adds a thin progress bar similar to the section progress
   bar underneath *each* slide.
 
+
+### Commands
+
+* `\plain{title=[]}{body}` sets a slice in plain dark colors.
+
+
 ### License
 
 The theme itself is licensed under a [Creative Commons Attribution-ShareAlike

beamerthemem.sty

@@ -129,7 +129,7 @@
   \newcommand{\insertsectionHEADaux}[3]{\textsc{\MakeLowercase{#3}}
 }
 
-\newcommand{\plain}[2]{%
+\newcommand{\plain}[2][]{%
   \begingroup
   \setbeamercolor{background canvas}{bg=mDarkTeal}
   \begin{frame}{#1}