Fix known issue with beamercolortheme fly

Previously `\plain` slides were broken if the user overrode the colour theme with `fly`, as that theme does not define a background colour for `palette primary`. The solution checks whether `palette primary.bg` is undefined; if so, it automatically picks a colour complementary to `palette primary.fg` for the background of the plain slides.

doc/metropolistheme.dtx

@@ -523,9 +523,6 @@ only alphabetic characters and do not require the expansion of any macros.
 
 
 
-\subsection{Plain Frame}
-The |\plain| command does not work if you override the \themename color theme
-with the default beamer color theme |fly|.
 
 
 

source/beamerthememetropolis.dtx

@@ -177,9 +177,16 @@
 \def\metropolis@plaintitleformat#1{#1}
 \newcommand{\plain}[2][]{%
   \begingroup
-    \setbeamercolor{background canvas}{
-      use=palette primary,
-      parent=palette primary
+    \ifbeamercolorempty[bg]{palette primary}{
+      \setbeamercolor{background canvas}{
+        use=palette primary,
+        bg=-palette primary.fg
+      }
+    }{
+      \setbeamercolor{background canvas}{
+        use=palette primary,
+        bg=palette primary.bg
+      }
     }
     \begin{frame}[c]{#1}
       \begin{center}