Mrthfx

Starting from this MWE,

    documentclass[12pt,fleqn]{book} 

    usepackage[top=3cm,bottom=3cm,left=3cm,right=3cm,headsep=10pt,a4paper]{geometry} % Page margins



    usepackage{graphicx} % Required for including pictures

    usepackage{tikz} % Required for drawing custom shapes

    usepackage{xcolor} % Required for specifying colors by name

    definecolor{ocre}{RGB}{243,102,25} % Define the orange color used for highlighting throughout the book



    %----------------------------------------------------------------------------------------

    %   SECTION NUMBERING IN THE MARGIN

    %----------------------------------------------------------------------------------------



    makeatletter

    renewcommand{@seccntformat}[1]{llap{textcolor{ocre}{csname the#1endcsname}hspace{1em}}}                    

    renewcommand{section}{@startsection{section}{1}{z@}

    {-4ex @plus -1ex @minus -.4ex}

    {1ex @plus.2ex }

    {normalfontlargesffamilybfseries}}

    renewcommand{subsection}{@startsection {subsection}{2}{z@}

    {-3ex @plus -0.1ex @minus -.4ex}

    {0.5ex @plus.2ex }

    {normalfontsffamilybfseries}}

    renewcommand{subsubsection}{@startsection {subsubsection}{3}{z@}

    {-2ex @plus -0.1ex @minus -.2ex}

    {.2ex @plus.2ex }

    {normalfontsmallsffamilybfseries}}                        

    renewcommandparagraph{@startsection{paragraph}{4}{z@}

    {-2ex @plus-.2ex @minus .2ex}

    {.1ex}

    {normalfontsmallsffamilybfseries}}





    %----------------------------------------------------------------------------------------

    %   CHANGE THIS STYLE CHAPTER

    %----------------------------------------------------------------------------------------



% A switch to conditionally include a picture, implemented by  Christian Hupfer

    newififusechapterimage

    usechapterimagetrue

    newcommand{thechapterimage}{}%

    newcommand{chapterimage}[1]{ifusechapterimagerenewcommand{thechapterimage}{#1}fi}%

    newcommand{autodot}{.}

    def@makechapterhead#1{%

    {parindent z@ raggedright normalfont

    ifnum c@secnumdepth >m@ne

    if@mainmatter

    begin{tikzpicture}[remember picture,overlay]

    node at (current page.north west)

    {begin{tikzpicture}[remember picture,overlay]

    node[anchor=north west,inner sep=0pt] at (0,0) {ifusechapterimageincludegraphics[width=paperwidth]{thechapterimage}fi};

    draw[anchor=west] (Gm@lmargin,-9cm) node [line width=2pt,rounded corners=15pt,draw=ocre,fill=white,fill opacity=0.5,inner sep=15pt]{strutmakebox[22cm]{}};

    draw[anchor=west] (Gm@lmargin+.3cm,-9cm) node {hugesffamilybfseriescolor{black}thechapterautodot~#1strut};

    end{tikzpicture}};

    end{tikzpicture}

    else

    begin{tikzpicture}[remember picture,overlay]

    node at (current page.north west)

    {begin{tikzpicture}[remember picture,overlay]

    node[anchor=north west,inner sep=0pt] at (0,0) {ifusechapterimageincludegraphics[width=paperwidth]{thechapterimage}fi};

    draw[anchor=west] (Gm@lmargin,-9cm) node [line width=2pt,rounded corners=15pt,draw=ocre,fill=white,fill opacity=0.5,inner sep=15pt]{strutmakebox[22cm]{}};

    draw[anchor=west] (Gm@lmargin+.3cm,-9cm) node {hugesffamilybfseriescolor{black}#1strut};

    end{tikzpicture}};

    end{tikzpicture}

    fifiparvspace*{270p@}}}



    %-------------------------------------------



    def@makeschapterhead#1{%

    begin{tikzpicture}[remember picture,overlay]

    node at (current page.north west)

    {begin{tikzpicture}[remember picture,overlay]

    node[anchor=north west,inner sep=0pt] at (0,0) {ifusechapterimageincludegraphics[width=paperwidth]{thechapterimage}fi};

    draw[anchor=west] (Gm@lmargin,-9cm) node [line width=2pt,rounded corners=15pt,draw=ocre,fill=white,fill opacity=0.5,inner sep=15pt]{strutmakebox[22cm]{}};

    draw[anchor=west] (Gm@lmargin+.3cm,-9cm) node {hugesffamilybfseriescolor{black}#1strut};

    end{tikzpicture}};

    end{tikzpicture}

    parvspace*{270p@}}

    makeatother

    begin{document}

    chapterimage{example-image-a}

    chapter{Text Chapter}

    section{Paragraphs of Text}index{Paragraphs of Text}

    end{document}

looking carefully at this code, there is a rounded rectangle, ochre color, with opacity that provides a good contrast with the image below.

Using the same identical structural code I am not able to operate a variant, with TikZ. In fact, to create that rounded rectangle where the text is present and the ochre border is used a code complex for me of TikZ.

I'd like to have something different of this shape. How can I get this same image, for example,

using the same identical code of my MWE operating the variant only of the graphic part?

I kindly ask for your precious help.

I still don't know if I get the message but this is something that is based on this answer. It does some orange boxes whose size adjusts to the multiline chapter titles. While the TeX code is not particularly interesting, the chapter titles contain a deep philosophical message, which is why I fully understand that some users may need styles that allow them to run chapter titles over several lines. ;-)

documentclass[12pt,fleqn]{book} 

usepackage[top=3cm,bottom=3cm,left=3cm,right=3cm,headsep=10pt,a4paper]{geometry} % Page margins

usepackage{lmodern}

usepackage[explicit]{titlesec}

usepackage{microtype}

usepackage{tikz}

usetikzlibrary{positioning,calc}

definecolor{myblue}{RGB}{0,82,155}

% from https://tex.stackexchange.com/a/169435/121799

titleformat{chapter}[display]

  {normalfontbfseriescolor{myblue}}

  {%filleft%

    begin{tikzpicture}

    node[inner xsep=0.5em,text 

    width={textwidth-2em-2.5cm},align=left,font=Largesffamily]

    (CC){#1};

    path let p1=($(CC.north)-(CC.south)$) in

    node[minimum width=2.5cm,minimum height={min(2.5cm,y1)},left=1em of CC,

    fill=orange,font=Hugesffamilybfseries,text=blue] (TC)    {thechapter};  

    %{chaptertitlename~thechapter};

    end{tikzpicture}%

  }

  {10pt}

  {}



begin{document}

pagestyle{empty}

chapter{Those who can hibernate on a Monday can also hibernate on a Tuesday.}

chapter{Those who can hibernate on a Monday can also hibernate on a Tuesday and

even on a Wednesday.}



end{document}