The whole thing using TikZ

After one commenter asked why I didn't do the whole titlepage in TikZ, I couldn't find rest until I've tried. Here is the result. I'm sure, the left aligned positioning in the center can be optimized. If anyone knows how, just drop me a line and I'll edit.

The code is a lot shorter and cleaner now:

documentclass{article}
usepackage{librebaskerville}
usepackage{tikz}
usetikzlibrary{positioning,fit,shapes.geometric,calc,backgrounds}

tikzstyle{titlepage}=[text width=paperwidth, text centered]

definecolor{titlepage-background}{RGB}{160, 180, 200}
definecolor{titlepage-title}{RGB}{26, 51, 74}
definecolor{titlepage-ampersand}{RGB}{188, 205, 219}
definecolor{titlepage-subtitle}{RGB}{97, 125, 150}

begin{document}
    begin{titlepage}
        
        begin{tikzpicture}[remember picture,overlay]
            fill[titlepage-background] (current page.south east) rectangle (current page.north west);
            
            draw[line width = 1in,color = white] ($(current page.north west) + (0.25in,0.25in)$) rectangle ($(current page.south east) + (0.25in,-0.25in)$);
            draw[line width = 0.25pt,color = black] ($(current page.north west) + (0.75in,-0.25in)$) rectangle ($(current page.south east) + (-0.25in,0.25in)$);
            draw[line width = 1pt,color = white] ($(current page.north west) + (1in,-0.5in)$) rectangle ($(current page.south east) + (-0.5in,0.5in)$);
            
            node [opacity=1,titlepage-ampersand,anchor=west] at (7.25,-3.5) {scalebox{10}{&}};
            node [titlepage-title,anchor=west,xshift=.725in] at (0,-3) {scalebox{4}{textbf{Matière}}};
            node [titlepage-title,anchor=west,xshift=.725in] at (0,-4.5) {scalebox{3}{textbf{Espace-Temps}}};
            node [titlepage-subtitle,anchor=west,xshift=.725in] (theorie) at (0,-6) {scalebox{1.1}{La théorie classique des champs relativistes}};
            node [titlepage, titlepage-subtitle] at ([xshift=.25in]current page.center) {Mickey Mouse};
            
            node[xshift=.25in, inner sep=0pt] (image) at ([yshift=7cm]current page.south) {includegraphics[width=8.5cm]{example-image}};
            node[align=left,black,anchor=north] at ([yshift=-1.5cm]image.south) {Version number};
        end{tikzpicture}

    end{titlepage}
end{document}

The output is quite nice, too:

The older messier solution

Well, the code certainly ain't pretty. But it is a start.

I personally think that what I've done defeats the purpose of using LaTeX since you have to manually fiddle with sizes, dimensions, and spacing as soon as you change the font family or the font size – the horror!

I justify my actions with the fact that you only have one title page, so this abomination I call code won't happen too often ;-)

I invite all the experts to chime in and edit away

documentclass[letterpaper]{book}

usepackage{librebaskerville}
usepackage[T1]{fontenc}

usepackage{microtype}
usepackage[french]{babel}
usepackage{anysize}
marginsize{1.5in}{1in}{0.5in}{0.5in}
usepackage[x11names]{xcolor}
definecolor{backgroundcolor}{RGB}{160,180,200}
definecolor{myamper}{RGB}{188, 205, 219}
definecolor{subtitle}{RGB}{97, 125, 150}
usepackage{graphicx}
usepackage{here}
usepackage{afterpage}
usepackage{tikz}
usetikzlibrary{calc}


newcommand*{maintitle}{begingroup

pagecolor{backgroundcolor}

centering

vspace*{5baselineskip}
begin{minipage}[t][5cm][b]{.5textwidth}
{fontsize{35}{30}selectfonttextbf{Matière}}

{raisebox{-2.25em}[0pt][0pt]{fontsize{95}{90}selectfontbfseriesnoindenthspace{1.35em}textcolor{myamper} &}}

{fontsize{25}{25}selectfonttextbf{Espace-Temps}}

vspace*{baselineskip}

textcolor{subtitle}{La théorie classique des champs relativistes}
end{minipage}
vfill

{Large Mickey Mouse}

vfill

    newlength{titlelength}
    settowidth{titlelength}{Hugetextbf{Matière & Espace-Temps}}

    begin{figure}[H]
        centering
        fbox{includegraphics[width=titlelength]{example-image}}
    end{figure}

vfill

{large Version number}

par

endgroup}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

begin{document}

begin{titlepage}

% FRAME AROUND PAGE :
    begin{tikzpicture}[remember picture, overlay]
        draw[line width = 1in,color = white] ($(current page.north west) + (0.25in,0.25in)$) rectangle ($(current page.south east) + (0.25in,-0.25in)$);
        draw[line width = 0.25pt,color = black] ($(current page.north west) + (0.75in,-0.25in)$) rectangle ($(current page.south east) + (-0.25in,0.25in)$);
        draw[line width = 1pt,color = white] ($(current page.north west) + (1in,-0.5in)$) rectangle ($(current page.south east) + (-0.5in,0.5in)$);
    end{tikzpicture}

    maintitle
end{titlepage}

end{document}