You can redefine the headline, footline and frametitle templates; something along these lines:
\documentclass{beamer}
\definecolor{secinhead}{RGB}{249,196,95}
\definecolor{titlebg}{RGB}{51,51,51}
\setbeamercolor{secsubsec}{fg=secinhead,bg=black}
\setbeamercolor{frametitle}{fg=secinhead,bg=titlebg}
\setbeamertemplate{headline}
{
\leavevmode%
\hbox{%
\begin{beamercolorbox}[wd=\paperwidth,ht=8.25ex,dp=3.5ex]{secsubsec}%
\raggedright
\hspace*{2em}%
{\sffamily\Large\color{secinhead}\thesection.~\insertsection\hfill\insertsubsection}%
\hspace*{2em}%
\end{beamercolorbox}%
}%
}
\setbeamertemplate{frametitle}
{\vskip-3pt
\leavevmode
\hbox{%
\begin{beamercolorbox}[wd=\paperwidth,ht=1.8ex,dp=1ex]{frametitle}%
\raggedright\hspace*{2em}\small\insertframetitle
\end{beamercolorbox}
}%
}
\setbeamertemplate{footline}{}
\begin{document}
\section{Introduction}
\subsection{The questions}
\begin{frame}\frametitle{A test frame}Test\end{frame}
\subsection{Another questions}
\begin{frame}\frametitle{Another test frame}Test\end{frame}
\end{document}
Note however that probably you will have to redefine also some other templates to mantain consistency; for example, which color/font to use to display the frame title? which color for enumerated lists?
Another option, this time the frametitle will have to be given another definition:
\documentclass{beamer}
\definecolor{secinhead}{RGB}{249,196,95}
\definecolor{shadowbg}{RGB}{51,51,51}
\setbeamercolor{secsubsec}{fg=secinhead,bg=black}
\setbeamercolor{shadow}{fg=secinhead,bg=shadowbg}
\setbeamertemplate{headline}
{
\leavevmode%
\hbox{%
\begin{beamercolorbox}[wd=\paperwidth,ht=8.25ex,dp=3.5ex]{secsubsec}%
\raggedright
\hspace*{2em}%
{\sffamily\Large\color{secinhead}\thesection.~\insertsection\hfill\insertsubsection}%
\hspace*{2em}%
\end{beamercolorbox}%
}\vskip0pt%
\hbox{%
\begin{beamercolorbox}[wd=\paperwidth,ht=1.8ex,dp=1ex]{shadow}%
\mbox{}
\end{beamercolorbox}
}%
}
\setbeamertemplate{frametitle}{}
\setbeamertemplate{footline}{}
\begin{document}
\section{Introduction}
\subsection{The questions}
\begin{frame}Test\end{frame}
\subsection{Another questions}
\begin{frame}Test\end{frame}
\end{document}
Here's the code producing the vertical shading color for the second vertical "bar":
\documentclass{beamer}
\usepackage{tikz}
\usetikzlibrary{shadings}
\definecolor{secinhead}{RGB}{249,196,95}
\definecolor{shadowbg}{RGB}{51,51,51}
\setbeamercolor{secsubsec}{fg=secinhead,bg=black}
\setbeamercolor{shadow}{fg=secinhead,bg=shadowbg}
\setbeamertemplate{headline}
{
\leavevmode%
\hbox{%
\begin{beamercolorbox}[wd=\paperwidth,ht=8.25ex,dp=3.5ex]{secsubsec}%
\raggedright
\hspace*{2em}%
{\sffamily\Large\color{secinhead}\thesection.~\insertsection\hfill\insertsubsection}%
\hspace*{2em}%
\end{beamercolorbox}%
}\vskip-1pt%
\hbox{%
\tikz\draw[draw=none,top color=black,bottom color=shadowbg!60] (0,0) rectangle (\paperwidth,0.5);
}%
}
\setbeamertemplate{frametitle}{}
\setbeamertemplate{footline}{}
\begin{document}
\section{Introduction}
\subsection{The questions}
\begin{frame}
Test
\end{frame}
\end{document}
and here's the upper part of the resulting frame:
Of course, change the values for top color, and bottom color according to your needs (you can even select a middle color value).
As said in the comments, the right way to proceed is to define separately the two templates headline and frametitle.
Here is a possible implementation (as test image I used the logo of my university):
\documentclass{beamer}
\usepackage{lmodern}
%=====================================================================
% Color definition
% Setzt Deep Harlequin (dunkler)
\definecolor{jvagreen}{RGB}{22, 89, 0}
% Setzt Vivid Gold (dunkler)
\definecolor{jvagold}{RGB}{231, 202, 0}
% Farbe der Elemente der headline
\setbeamercolor{section in head/foot}{fg = jvagold, bg = jvagreen}
%=====================================================================
% Templates - headline, frametitle
\makeatletter
% Komprimiert die miniframe Kreise auf eine Linie
\beamer@compresstrue
\makeatother
% Definiert die headline
\setbeamertemplate{headline}
{
\includegraphics[width=0.15\paperwidth]{logopolito} % test logo
\begin{beamercolorbox}[wd=.85\paperwidth,right]{section in head/foot}
\rule{0.85\paperwidth}{1pt}
%Vertikaler Abstand
\vskip10pt
%Fügt die Standard-Navi ein (miniframes)
\insertnavigation{0.85\paperwidth}
\vskip8pt
\rule{0.85\paperwidth}{0.5pt}
\vskip27.5pt % same height of the example provided, but IMHO is too much
\rule{0.85\paperwidth}{1pt}
\end{beamercolorbox}
}
% definition of the frametitle
\setbeamertemplate{frametitle}
{
\vskip-24pt % to shift up the frametitle
\hbox{
\begin{beamercolorbox}[wd=.0675\textwidth]{} % left shift
\end{beamercolorbox}
\begin{beamercolorbox}[sep=4pt]{section in head/foot}
\insertframetitle
\end{beamercolorbox}
}
}
\begin{document}
\section{Test 1}
\subsection{first}
\begin{frame}{A title}
hello
\end{frame}
\subsection{second}
\begin{frame}{A second title}
hello
\end{frame}
\section{Test 2}
\subsection{first}
\begin{frame}{Another title}
hello
\end{frame}
\subsection{second}
\begin{frame}{A second title}
hello
\end{frame}
\section{Test 3}
\subsection{first}
\begin{frame}{Last title}
hello
\end{frame}
\subsection{second}
\begin{frame}{A second title}
hello
\end{frame}
\subsection{second}
\begin{frame}{A third title}
hello
\end{frame}
\end{document}
The final result is:
Best Answer
To change the headline background color in the Ilmenau theme we need to re set the
section in head/footcolor like this:the same macro can be used for the text color too: