Color transition from title to main body

\documentclass{beamer}
\usetheme{Ilmenau}
\useoutertheme[subsection=false]{smoothbars}

\usepackage{lipsum}

\setbeamertemplate{footline}[default]
\setbeamertemplate{navigation symbols}{}

\definecolor{color3}{rgb}{.4 .4 .4}

\usecolortheme[named = color3]{structure}
\setbeamercolor{frametitle}{fg = white}
\setbeamercolor{background canvas}{bg = black}
\setbeamercolor{normal text}{fg = white}

\makeatletter
\AtBeginDocument{
  {
    \pgfdeclareverticalshading{beamer@belowframetitle}{\the\paperwidth}{%
      color(0ex)=(black);%(global.bg);%
      color(1ex)=(frametitle.bg)
    }
  }
}
\makeatother

\begin{document}
\begin{frame}
    \frametitle{Test}
    \framesubtitle{Test framesubtitle}
    \lipsum[4]
\end{frame}
\end{document}