Let's assume the so called shaded background be placed in the headline
template.
In this case, the answer provides a method to insert a vertical shaded background or a simple background.
Simple background
\documentclass[xcolor=dvipsnames,14pt]{beamer}
\usepackage[english]{babel}
\usepackage[latin1]{inputenc}
\usepackage{tikz}
\usetikzlibrary{positioning,shapes}
\usetikzlibrary{calc}
\setlength\fboxsep{0pt}
\setlength\fboxrule{0.5pt}
\setlength\parskip{0.75em}
\setlength\parindent{0pt}
\setbeamersize{text margin left=3mm,text margin right=3mm}{}
\setbeamerfont{section in head/foot}{size=\scriptsize}
\setbeamerfont{subsection in head/foot}{size=\scriptsize}
\setbeamerfont{date in head/foot}{size=\scriptsize}
\setbeamercolor{section in head/foot}{fg=black,bg=gray}
\setbeamercolor{subsection in head/foot}{fg=black,bg=gray}
\setbeamercolor{date in head/foot}{fg=black,bg=gray}
% uniform color
\setbeamercolor{shadedhead}{bg=blue}
\setbeamertemplate{headline}{
\leavevmode\hbox{%
\begin{beamercolorbox}[wd=54mm,ht=3ex,dp=1ex,right]{section in head/foot}%
\usebeamerfont{section in head/foot}\insertsectionhead\hspace*{2ex}%
\end{beamercolorbox}%
\begin{beamercolorbox}[wd=65mm,ht=3ex,dp=1ex,right]{subsection in head/foot}%
\usebeamerfont{subsection in head/foot}\insertsubsectionhead\hspace*{2ex}%
\end{beamercolorbox}%
\begin{beamercolorbox}[wd=9mm,ht=3ex,dp=1ex,right]{date in head/foot}%
\usebeamerfont{date in head/foot}\insertframenumber{}\hspace*{2ex}%
\end{beamercolorbox}%
}\vskip0pt
\leavevmode\hbox{%
% uniform
\begin{beamercolorbox}[wd=\textwidth,ht=3ex,dp=2ex]{shadedhead}%
\end{beamercolorbox}%
}\vskip0pt
}
\begin{document}
\section{Section title}
\subsection{Subsection title}
\begin{frame}
\frametitle{Frame title}
some text
\end{frame}
\begin{frame}
\frametitle{Frame title 2}
some text
\end{frame}
\subsection{Subsection second title}
\begin{frame}
\frametitle{Frame title}
some text
\end{frame}
\begin{frame}
\frametitle{Frame title 2}
some text
\end{frame}
\end{document}
The background has been inserted by means of:
\leavevmode\hbox{%
% uniform
\begin{beamercolorbox}[wd=\textwidth,ht=3ex,dp=2ex]{shadedhead}%
\end{beamercolorbox}%
}
and the color definition could be modified by changing:
\setbeamercolor{shadedhead}{bg=blue}
You can adapt the dimensions in the options of the beamercolorbox
environment.
Two frames of the result:


Background with vertical shading
Let's define as first the shading:
% Shade definition
\pgfdeclareverticalshading{mytopshade}{\paperwidth}{%
color(0pt)=(gray!40);
color(10pt)=(gray)}
You can find further informations on the possible shadings in the pgfmanual (also related How to create a ball shading and to customize 3D lighting manually?).
Now we apply this shading in the headline
template:
\leavevmode\hbox{%
\begin{beamercolorbox}[wd=\textwidth,ht=3ex,dp=2ex]{}%
\begin{pgfpicture}{0pt}{0pt}{\paperwidth}{3ex}
\pgfpathrectangle{\pgfpointorigin}{\pgfpoint{\paperwidth}{3ex}}
\pgfusepath{clip}
\pgftext[left,base]{\pgfuseshading{mytopshade}}
\end{pgfpicture}
\end{beamercolorbox}%
}
The whole code:
\documentclass[xcolor=dvipsnames,14pt]{beamer}
\usepackage[english]{babel}
\usepackage[latin1]{inputenc}
\usepackage{tikz}
\usetikzlibrary{positioning,shapes}
\usetikzlibrary{calc}
\setlength\fboxsep{0pt}
\setlength\fboxrule{0.5pt}
\setlength\parskip{0.75em}
\setlength\parindent{0pt}
\setbeamersize{text margin left=3mm,text margin right=3mm}{}
\setbeamerfont{section in head/foot}{size=\scriptsize}
\setbeamerfont{subsection in head/foot}{size=\scriptsize}
\setbeamerfont{date in head/foot}{size=\scriptsize}
\setbeamercolor{section in head/foot}{fg=black,bg=gray}
\setbeamercolor{subsection in head/foot}{fg=black,bg=gray}
\setbeamercolor{date in head/foot}{fg=black,bg=gray}
% Shade definition
\pgfdeclareverticalshading{mytopshade}{\paperwidth}{%
color(0pt)=(gray!40);
color(10pt)=(gray)}
\setbeamertemplate{headline}{
\leavevmode\hbox{%
\begin{beamercolorbox}[wd=54mm,ht=3ex,dp=1ex,right]{section in head/foot}%
\usebeamerfont{section in head/foot}\insertsectionhead\hspace*{2ex}%
\end{beamercolorbox}%
\begin{beamercolorbox}[wd=65mm,ht=3ex,dp=1ex,right]{subsection in head/foot}%
\usebeamerfont{subsection in head/foot}\insertsubsectionhead\hspace*{2ex}%
\end{beamercolorbox}%
\begin{beamercolorbox}[wd=9mm,ht=3ex,dp=1ex,right]{date in head/foot}%
\usebeamerfont{date in head/foot}\insertframenumber{}\hspace*{2ex}%
\end{beamercolorbox}%
}\vskip0pt
\leavevmode\hbox{%
\begin{beamercolorbox}[wd=\textwidth,ht=3ex,dp=2ex]{}%
\begin{pgfpicture}{0pt}{0pt}{\paperwidth}{3ex}
\pgfpathrectangle{\pgfpointorigin}{\pgfpoint{\paperwidth}{3ex}}
\pgfusepath{clip}
\pgftext[left,base]{\pgfuseshading{mytopshade}}
\end{pgfpicture}
\end{beamercolorbox}%
}\vskip0pt
}
\begin{document}
\section{Section title}
\subsection{Subsection title}
\begin{frame}
\frametitle{Frame title}
some text
\end{frame}
\begin{frame}
\frametitle{Frame title 2}
some text
\end{frame}
\subsection{Subsection second title}
\begin{frame}
\frametitle{Frame title}
some text
\end{frame}
\begin{frame}
\frametitle{Frame title 2}
some text
\end{frame}
\end{document}
Now the same frames of the previous example are diplayed as:

