\documentclass{beamer}
\useoutertheme{bgblocks}

%% Define simple blocks: an X in the lower-left corner, and a label nearby;
%% and a large light-green disc centered on the top-left corner (so that
%% only a quarter of the disc is visible).
\defbgblock{x=0pt, y=0pt}{\Huge X}
\defbgblock{x=1ex, y=0pt, above right}{$\leftarrow$ X at lower left corner}
\defbgblock{x=0pt, y=\bgtextblockheight}{%
  \resizebox{8cm}{8cm}{%
    \textcolor{green!50!white}{$\bullet$}%
  }%
}

%% Slightly more complex block: a red box whose top and right borders are 1mm
%% inside the top-right corner (requires package calc).  Use a block name so
%% as to demonstrate setting font and color.  Block is in the foreground.
\usepackage{calc}
\defbgblock[named block upper right]{%
  x=\bgtextblockwidth-1mm,
  y=\bgtextblockheight-1mm,
  foreground,
  below left}{%
  \begin{beamercolorbox}
    [sep=2em, wd=10em, center]{\bgblocktemplate{named block upper right}}
    Red box at upper right (foreground)
  \end{beamercolorbox}%
}
\setbeamercolor{\bgblocktemplate{named block upper right}}{bg=red}
\setbeamerfont{\bgblocktemplate{named block upper right}}
              {size=\small,family=\itshape}

%% Likewise light-green box at center right, in the background.
\defbgblock[named block center right]{%
  x=\bgtextblockwidth-1mm,
  y=0.5\bgtextblockheight,
  background,
  left}{%
  \begin{beamercolorbox}
    [sep=2em, wd=10em, center]{\bgblocktemplate{named block center right}}
    Green box at center right (background)
  \end{beamercolorbox}%
}
\setbeamercolor{\bgblocktemplate{named block center right}}{bg=green!50!white}
\setbeamerfont{\bgblocktemplate{named block center right}}
              {size=\small,family=\itshape}

%% Meta-information, then the document itself.
\title{The title}
\author{C\'edric Ware}

\begin{document}
{
  % Disable blocks temporarily for the title page.
  \disablebgblocks
  \maketitle
}

%% Normal frame.
\begin{frame}{Frame title}
Foo
\end{frame}

%% Change frame: add a grid background and a colored-band frame title.
%% One must call \setupbgblocks to recalculate block positions.
\setbeamertemplate{background}[grid]
\setbeamercolor{frametitle}%
               {bg=blue, fg=white}
\setupbgblocks
\begin{frame}{Frame title + background}
Bar
\end{frame}

\end{document}
