title: Define a single background with beamer in LaTex
body: <p>I am using the Singapore theme in Beamer for multiple reasons (this theme is kind of clean, without too much vertical/horizontal bar everywhere, progress dots, ...) and so, I would like to keep it.</p>
  <p>Anyway, I defined a back ground with tikz with a circular color gradient background for slide content. But, the header is defined with a background too and I can't suceed to remove its background (even after 2 hours in internet lol).</p>
  <p>Here is a mwe where you can see the difference between the two background which make an horizontal white line that I do not want.</p>
  <pre><code>\documentclass{beamer}
  \usepackage[utf8]{inputenc}
  \usepackage{tikz}
  \usepackage{lipsum}
  
  \usetheme{Singapore}
  
  \begin{document}
  \usebackgroundtemplate{
    \begin{tikzpicture}
      \path [outer color = blue!5, inner color = blue!1]
        (0,0) rectangle (\paperwidth,\paperheight);
    \end{tikzpicture}
  }
  
  \section{Section}
  \begin{frame}
  \lipsum[1]   
  \end{frame}
  
  \end{document}
  </code></pre>
  <p><a href="https://i.stack.imgur.com/ierf3.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ierf3.png" alt="" /></a></p>
  <p>How can I do to make the background with a circular color gradient for the fullsize of the slide ? Without interruption of the header ?</p>