Ok, this method works:
1° Put between opening and closing braces everything concerning the frame(s) for which you want the theme to be modified. Do any modifications you want (background color, etc) there.
2° Redefine the frametitle template (I use the Frankfurt theme, so the original definitions are coming from the smoothbars theme, see the beamerthemeFrankfurt.sty file).
To do that, I have copy pasted the original definition of the frametitle template directly in my .tex file, where I needed it to be changed, and removed parts where shading was included:
%%%Copy-paste from /usr/share/texmf/tex/latex/beamer/themes/outer/beamerouterthemesmoothbars.sty:
%%%%\vskip.5ex %REMOVE THIS SPACE, WHICH IS DEDICATED TO SHADING, DEFINED BELOW (OPTIONAL)
%%%% SHADING DEF:
It might not be the best way to achieve this but the result is exactly what I needed.
To improve it, one should look in the direction of ESultanik's answer, once "titlepage" replaced by "frametitle". (It does work for the titlepage).
does something similar to what is needed but it looks very different from what Frankfurt theme without shadowing looks like. There is no straightforward way to apply changes/options to
because it doesn't accept any. An other way could be to start from the default theme and apply laboriously every modification made by the theme (painful, the above solution looks easier).
ESultanik, you have confused the title of a frame and the titlepage. Nevertheless, it is your answer which made me find a working solution (though maybe not the best one).
vanden: this is true but I have used a temporary account so I could not add comments, after cleaning cookies, even for my own question.