Gdtyjr

Starting from this MWE,

 documentclass[12pt,fleqn]book 
 usepackage[top=3cm,bottom=3cm,left=3cm,right=3cm,headsep=10pt,a4paper]geometry % Page margins

 usepackagegraphicx % Required for including pictures
 usepackagetikz % Required for drawing custom shapes
 usepackagexcolor % Required for specifying colors by name
 definecolorocreRGB243,102,25 % Define the orange color used for highlighting throughout the book

 %----------------------------------------------------------------------------------------
 % SECTION NUMBERING IN THE MARGIN
 %----------------------------------------------------------------------------------------

 makeatletter
 renewcommand@seccntformat[1]llaptextcolorocrecsname the#1endcsnamehspace1em 
 renewcommandsection@startsectionsection1z@
 -4ex @plus -1ex @minus -.4ex
 1ex @plus.2ex 
 normalfontlargesffamilybfseries
 renewcommandsubsection@startsection subsection2z@
 -3ex @plus -0.1ex @minus -.4ex
 0.5ex @plus.2ex 
 normalfontsffamilybfseries
 renewcommandsubsubsection@startsection subsubsection3z@
 -2ex @plus -0.1ex @minus -.2ex
 .2ex @plus.2ex 
 normalfontsmallsffamilybfseries 
 renewcommandparagraph@startsectionparagraph4z@
 -2ex @plus-.2ex @minus .2ex
 .1ex
 normalfontsmallsffamilybfseries


 %----------------------------------------------------------------------------------------
 % CHANGE THIS STYLE CHAPTER
 %----------------------------------------------------------------------------------------

% A switch to conditionally include a picture, implemented by Christian Hupfer
 newififusechapterimage
 usechapterimagetrue
 newcommandthechapterimage%
 newcommandchapterimage[1]ifusechapterimagerenewcommandthechapterimage#1fi%
 newcommandautodot.
 def@makechapterhead#1%
 parindent z@ raggedright normalfont
 ifnum c@secnumdepth >m@ne
 if@mainmatter
 begintikzpicture[remember picture,overlay]
 node at (current page.north west)
 begintikzpicture[remember picture,overlay]
 node[anchor=north west,inner sep=0pt] at (0,0) ifusechapterimageincludegraphics[width=paperwidth]thechapterimagefi;
 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 hugesffamilybfseriescolorblackthechapterautodot~#1strut;
 endtikzpicture;
 endtikzpicture
 else
 begintikzpicture[remember picture,overlay]
 node at (current page.north west)
 begintikzpicture[remember picture,overlay]
 node[anchor=north west,inner sep=0pt] at (0,0) ifusechapterimageincludegraphics[width=paperwidth]thechapterimagefi;
 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 hugesffamilybfseriescolorblack#1strut;
 endtikzpicture;
 endtikzpicture
 fifiparvspace*270p@

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

 def@makeschapterhead#1%
 begintikzpicture[remember picture,overlay]
 node at (current page.north west)
 begintikzpicture[remember picture,overlay]
 node[anchor=north west,inner sep=0pt] at (0,0) ifusechapterimageincludegraphics[width=paperwidth]thechapterimagefi;
 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 hugesffamilybfseriescolorblack#1strut;
 endtikzpicture;
 endtikzpicture
 parvspace*270p@
 makeatother
 begindocument
 chapterimageexample-image-a
 chapterText Chapter
 sectionParagraphs of TextindexParagraphs of Text
 enddocument

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
usepackagelmodern
usepackage[explicit]titlesec
usepackagemicrotype
usepackagetikz
usetikzlibrarypositioning,calc
definecolormyblueRGB0,82,155
% from https://tex.stackexchange.com/a/169435/121799
titleformatchapter[display]
 normalfontbfseriescolormyblue
 %filleft%
 begintikzpicture
 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;
 endtikzpicture%
 
 10pt
 

begindocument
pagestyleempty
chapterThose who can hibernate on a Monday can also hibernate on a Tuesday.
chapterThose who can hibernate on a Monday can also hibernate on a Tuesday and
even on a Wednesday.

enddocument