Base block spacing on bg colour, not theme option

doc/metropolistheme.dtx

@@ -357,10 +357,6 @@ The list of options is structured as shown in the following example.
 
 
 \subsubsection{Inner theme}
-\DescribeOption{block}{transparent, fill}{transparent}{
-  Optionally adds a light grey background to block environments like |theorem|
-  and |example|.
-}
 
 \DescribeOption{sectionpage}{none, simple, progressbar}{progressbar}{
   Adds a slide at the start of each section (|simple|) with an optional thin
@@ -416,6 +412,7 @@ terms of three beamer colors:
     \item |alerted text| (colored fg, should be visible against dark or light)
     \item |example text| (colored fg, should be visible against dark or light)
 \end{itemize}
+
 An easy way to customize the theme is to redefine these colors using
 
 \begin{lstlisting}

source/beamercolorthememetropolis.dtx

@@ -41,7 +41,8 @@
 % \subsubsection{Options}
 %
 % \begin{macro}{block}
-%    Controls whether block environments are filled or transparent.
+%    Optionally adds a light grey background to block environments like
+%    |theorem| and |example|.
 %    \begin{macrocode}
 \pgfkeys{
   /metropolis/color/block/.cd,
@@ -176,11 +177,22 @@
 }
 %    \end{macrocode}
 %
-% Blocks
+% Block environments such as |theorem| and |example| have no background color
+% by default. The option |block=fill| sets a background color based on the
+% background and foreground of |normal text|. The option |block=transparent|
+% reverts the block environments to an empty background, which can be useful
+% if changing colors mid-presentation.
 %
 %    \begin{macrocode}
 \newcommand{\metropolis@block@transparent}{
-  \setbeamercolor{block title}{use=normal text, parent=normal text}
+  \setbeamercolor{block title}{%
+    use=normal text,
+    fg=normal text.fg,
+    bg=
+  }
+  \setbeamercolor{block body}{
+    bg=
+  }
 }
 \newcommand{\metropolis@block@fill}{
   \setbeamercolor{block title}{%
@@ -188,6 +200,10 @@
     fg=normal text.fg,
     bg=normal text.bg!80!fg
   }
+  \setbeamercolor{block body}{
+    use={block title, normal text},
+    bg=block title.bg!50!normal text.bg
+  }
 }
 \setbeamercolor{block title alerted}{%
     use={block title, alerted text},
@@ -201,10 +217,6 @@
 }
 \setbeamercolor{block body alerted}{use=block body, parent=block body}
 \setbeamercolor{block body example}{use=block body, parent=block body}
-\setbeamercolor{block body}{
-  use={block title, normal text},
-  bg=block title.bg!50!normal text.bg
-}
 %    \end{macrocode}
 %
 % Footnotes

source/beamerinnerthememetropolis.dtx

@@ -55,18 +55,6 @@
 %
 % \subsubsection{Options}
 %
-% \begin{macro}{block}
-% This option controls the block style.
-%    \begin{macrocode}
-\pgfkeys{
-  /metropolis/inner/block/.cd,
-    .is choice,
-    transparent/.code=\setlength{\metropolis@blockskip}{0ex},
-    fill/.code=\setlength{\metropolis@blockskip}{1ex},
-}
-%    \end{macrocode}
-% \end{macro}
-%
 % \begin{macro}{sectionpage}
 % The |sectionpage| option defines the behaviour of the sectionpage.
 %    \begin{macrocode}
@@ -86,7 +74,6 @@
 \newcommand{\metropolis@inner@setdefaults}{
   \pgfkeys{/metropolis/inner/.cd,
     sectionpage=progressbar,
-    block=transparent,
   }
 }
 %    \end{macrocode}
@@ -328,87 +315,70 @@
 %
 % \subsubsection{Block environments}
 %
-% Regular block environment
+%
+% \begin{macro}{block}
+% \begin{macro}{block alerted}
+% \begin{macro}{block example}
+%
+%    The three different block environments differ only in their colours.
+%    Rather than repeat the essentially the same template three times, we use
+%    the auxiliary macro |\metropolis@block| to define all three templates.
 %
 %    \begin{macrocode}
-\newlength{\metropolis@blockskip}
-\setbeamertemplate{block begin}{%
+\newlength{\metropolis@blocktitleskip}
+\newlength{\metropolis@blockbodyskip}
+\newcommand{\metropolis@block}[1]{
   \setlength{\parskip}{\metropolis@parskip}
+%    \end{macrocode}
+%
+%    If a background color is defined for the block title or body, a little
+%    bit of padding is added to the corresponding box.
+%
+%    \begin{macrocode}
+  \ifbeamercolorempty[bg]{block title}
+    {\setlength{\metropolis@blocktitleskip}{0ex}}
+    {\setlength{\metropolis@blocktitleskip}{1ex}}
+  \ifbeamercolorempty[bg]{block body}
+    {\setlength{\metropolis@blockbodyskip}{0ex}}
+    {\setlength{\metropolis@blockbodyskip}{1ex}}
   \vspace*{1ex}
+%    \end{macrocode}
+%
+%    Each block environment consists of two (possibly coloured) beamer boxes:
+%    one for the title and one for the body.
+%
+%    \begin{macrocode}
   \begin{beamercolorbox}[%
     ht=2.4ex,
     dp=1ex,
-    leftskip=\metropolis@blockskip,
-    rightskip=\metropolis@blockskip]{block title}
-      \usebeamerfont*{block title}\insertblocktitle%
+    leftskip=\metropolis@blocktitleskip,
+    rightskip=\metropolis@blocktitleskip]{block title#1}
+      \usebeamerfont*{block title#1}\insertblocktitle%
   \end{beamercolorbox}%
-  \vspace*{-1pt}
-  \usebeamerfont{block body}%
+  \nointerlineskip%
+  \usebeamerfont{block body#1}%
   \begin{beamercolorbox}[%
     dp=1ex,
-    leftskip=\metropolis@blockskip,
-    rightskip=\metropolis@blockskip,
-    vmode]{block body}%
-}
-\setbeamertemplate{block end}{%
-  \end{beamercolorbox}
-  \vspace*{0.2ex}
+    leftskip=\metropolis@blockbodyskip,
+    rightskip=\metropolis@blockbodyskip,
+    vmode]{block body#1}%
 }
 %    \end{macrocode}
 %
-% Alerted block environment
+%    This concludes the auxiliary macro |\metropolis@block|. Next,
+%    we define the block beamer templates using this macro.
 %
 %    \begin{macrocode}
-\setbeamertemplate{block alerted begin}{%
-  \setlength{\parskip}{\metropolis@parskip}
-  \vspace*{1ex}
-  \begin{beamercolorbox}[%
-    ht=2.4ex,
-    dp=1ex,
-    leftskip=\metropolis@blockskip,
-    rightskip=\metropolis@blockskip]{block title alerted}
-      \usebeamerfont*{block title alerted}\insertblocktitle%
-  \end{beamercolorbox}%
-  \vspace*{-1pt}
-  \usebeamerfont{block body alerted}%
-  \begin{beamercolorbox}[%
-    dp=1ex,
-    leftskip=\metropolis@blockskip,
-    rightskip=\metropolis@blockskip,
-    vmode]{block body alerted}%
-}
-\setbeamertemplate{block alerted end}{%
-  \end{beamercolorbox}
-  \vspace*{0.2ex}
-}
-%    \end{macrocode}
-%
-% Example block environment
-%
-%    \begin{macrocode}
-\setbeamertemplate{block example begin}{%
-  \setlength{\parskip}{\metropolis@parskip}
-  \vspace*{1ex}
-  \begin{beamercolorbox}[%
-    ht=2.4ex,
-    dp=1ex,
-    leftskip=\metropolis@blockskip,
-    rightskip=\metropolis@blockskip]{block title example}
-      \usebeamerfont*{block title example}\insertblocktitle%
-  \end{beamercolorbox}%
-  \vspace*{-1pt}
-  \usebeamerfont{block body example}%
-  \begin{beamercolorbox}[%
-    dp=1ex,
-    leftskip=\metropolis@blockskip,
-    rightskip=\metropolis@blockskip,
-    vmode]{block body example}%
-}
-\setbeamertemplate{block example end}{%
-  \end{beamercolorbox}
-  \vspace*{0.2ex}
-}
+\setbeamertemplate{block begin}{\metropolis@block{}}
+\setbeamertemplate{block alerted begin}{\metropolis@block{ alerted}}
+\setbeamertemplate{block example begin}{\metropolis@block{ example}}
+\setbeamertemplate{block end}{\end{beamercolorbox}\vspace*{0.2ex}}
+\setbeamertemplate{block alerted end}{\end{beamercolorbox}\vspace*{0.2ex}}
+\setbeamertemplate{block example end}{\end{beamercolorbox}\vspace*{0.2ex}}
 %    \end{macrocode}
+% \end{macro}
+% \end{macro}
+% \end{macro}
 %
 %
 %

source/beamerthememetropolis.dtx

@@ -55,17 +55,7 @@
     /metropolis/outer,
     /metropolis/color,
     /metropolis/font,
-  },
-%    \end{macrocode}
-%
-% Currently, the |block| option affects two subthemes and has to be handled
-% separately.
-%
-%    \begin{macrocode}
-  block/.code=\pgfkeysalso{
-    inner/block=#1,
-    color/block=#1,
-  },
+  }
 }
 %    \end{macrocode}
 %