Avec l'extension `tikz` on peut faire quelque chose comme ça :
\documentclass{beamer}
\usepackage{tikz}
\usetikzlibrary{shadings}
\usetikzlibrary{fadings}
\colorlet{frametitleleft}{red}
\colorlet{frametitleright}{blue}
\makeatletter
\pgfdeclarehorizontalshading{titlefont}{\paperheight}{%
color(0cm)=(frametitleleft);
color(2cm)=(frametitleright)
}
% from https://tex.stackexchange.com/questions/45744/how-to-put-color-gradient-to-desired-text-only
\newcommand\shadetext[1]{%
\begin{tikzfadingfrompicture}[name=tikzsection]
\node[text=white,inner sep=0pt,outer sep=0pt,text width=.9\linewidth] {#1};
\end{tikzfadingfrompicture}%
\begin{tikzpicture}%
\node[text=white,inner sep=0pt,outer sep=0pt,text width=.9\linewidth,opacity=0] (textnode) {#1};
\shade[path fading=tikzsection,fit fading=false,shading=titlefont,text width=.9\linewidth,inner sep=0pt,outer sep=0pt]
(textnode.south west) rectangle (textnode.north east);
\end{tikzpicture}%
}
\setbeamertemplate{frametitle}{%
\ifbeamercolorempty[bg]{frametitle}{}{\nointerlineskip}%
\@tempdima=\textwidth%
\advance\@tempdima by\beamer@leftmargin%
\advance\@tempdima by\beamer@rightmargin%
\begin{beamercolorbox}[sep=0.3cm,left,wd=\the\@tempdima]{frametitle}
\usebeamerfont{frametitle}%
\vbox{}\vskip-1ex%
\if@tempswa\else\csname beamer@fteleft\endcsname\fi%
\shadetext{\strut\insertframetitle}\par%
{%
\ifx\insertframesubtitle\@empty%
\else%
{\usebeamerfont{framesubtitle}\usebeamercolor[fg]{framesubtitle}\strut\insertframesubtitle\par}%
\fi
}%
\vskip-1ex%
\if@tempswa\else\vskip-.3cm\fi% set inside beamercolorbox... evil here...
\end{beamercolorbox}%
}
\makeatother
\begin{document}
\begin{frame}
\frametitle{Very very long frame title reaching the end of the line and continuing in the next line}
\end{frame}
\end{document}
![alt text][1]
-----
On peut même combiner cela avec un fond en dégradés de couleurs des dégradés :
\documentclass{beamer}
\useoutertheme{shadow}
\usepackage{tikz}
\usetikzlibrary{shadings}
\usetikzlibrary{fadings}
\colorlet{titleleft}{yellow!30!white}
\colorlet{titleright}{blue!20!black}
\makeatletter
\pgfdeclarehorizontalshading[titleleft,titleright]
{beamer@frametitleshade}
{\paperheight}{%
color(0pt)=(titleleft);
color(\paperwidth)=(titleright)
}
\pgfdeclarehorizontalshading{titlefont}
{\paperheight}
{%
color(0cm)=(titleright);
color(0.8cm)=(titleright);
color(1.15cm)=(blue);
color(1.65cm)=(blue!50!cyan);
color(2cm)=(blue!10!cyan)
}
% from https://tex.stackexchange.com/questions/45744/how-to-put-color-gradient-to-desired-text-only
\newcommand\shadetext[1]{%
\begin{tikzfadingfrompicture}[name=tikzsection]
\node [text=white,inner sep=0pt,outer sep=0pt,text width=.9\linewidth] {#1};
\end{tikzfadingfrompicture}%
\begin{tikzpicture}%
\node [text=white,inner sep=0pt,outer sep=0pt,text width=.9\linewidth,opacity=0] (textnode) {#1};
\shade[path fading=tikzsection,fit fading=false,shading=titlefont,text width=.9\linewidth,inner sep=0pt,outer sep=0pt]
(textnode.south west) rectangle (textnode.north east);
\end{tikzpicture}%
}
\setbeamertemplate{frametitle}{%
\nointerlineskip%
\vskip-2pt%
\hbox{\leavevmode
\advance\beamer@leftmargin by -12bp%
\advance\beamer@rightmargin by -12bp%
\beamer@tempdim=\textwidth%
\advance\beamer@tempdim by \beamer@leftmargin%
\advance\beamer@tempdim by \beamer@rightmargin%
\hskip-\Gm@lmargin\hbox{%
\setbox\beamer@tempbox=\hbox{\begin{minipage}[b]{\paperwidth}%
\vbox{}
\vskip-0.0ex%
\leftskip0.3cm%
\rightskip0.3cm plus1fil\leavevmode
\usebeamercolor[fg]{frametitle}\shadetext{\usebeamerfont{frametitle}\strut\insertframetitle\strut}\par%
\ifx\insertframesubtitle\@empty\else%
{\usebeamerfont*{framesubtitle}{\usebeamercolor[fg]{framesubtitle}\insertframesubtitle}\strut\par}%
\fi%
\nointerlineskip
\vbox{}%
\end{minipage}}%
\beamer@tempdim=\ht\beamer@tempbox%
\advance\beamer@tempdim by 4pt%
\begin{pgfpicture}{0pt}{0pt}{\paperwidth}{\beamer@tempdim}
\usebeamercolor{frametitle right}
\pgfpathrectangle{\pgfpointorigin}{\pgfpoint{\paperwidth}{\beamer@tempdim}}
\pgfusepath{clip}
\pgftext[left,base]{\pgfuseshading{beamer@frametitleshade}}
\end{pgfpicture}
\hskip-\paperwidth%
\box\beamer@tempbox%
}%
\hskip-\Gm@rmargin%
}%
\nointerlineskip
\vskip-0.2pt
\hbox to\textwidth{\hskip-\Gm@lmargin\pgfuseshading{beamer@topshade}\hskip-\Gm@rmargin}
\vskip-2pt
}
\makeatother
\begin{document}
\begin{frame}
\frametitle{Very very long frame title reaching the end of the line and continuing in the next line}
\end{frame}
\end{document}
![alt text][2]
[1]: https://texnique.fr/osqa/upfiles/Screen_Shot_2019-05-14_at_23.59.44.png
[2]: https://texnique.fr/osqa/upfiles/Screen_Shot_2019-05-15_at_00.04.53.png