\documentclass[10pt]{book}
\usepackage[margin=0.5in]{geometry} % for PAPER & MARGIN
\usepackage[many]{tcolorbox}    	% for COLORED BOXES (tikz and xcolor included)
\usepackage{mathspec} 			    % for FONTS
\usepackage{setspace}               % for LINE SPACING
\usepackage{multicol}               % for MULTICOLUMNS
\setmainfont{Noto Sans}[
    Kerning = On,
    Mapping = tex-text,
    Numbers = Uppercase, 
    BoldFont = Noto Sans SemiBold
]                           % setting the font as Noto Sans
\setlength\parindent{0pt}   % killing indentation for all the text
\setstretch{1.3}            % setting line spacing to 1.3
\setlength\columnsep{0.25in} % setting length of column separator
\pagestyle{empty}           % setting pagestyle to be empty
\definecolor{main}{HTML}{5989cf}    % setting main color to be used
\definecolor{sub}{HTML}{cde4ff}     % setting sub color to be used
\tcbset{
    sharp corners,
    colback = white,
    before skip = 0.2cm,    % add extra space before the box
    after skip = 0.5cm      % add extra space after the box
}                           % setting global options for tcolorbox
% You can copy any following box you like to your code.
\newtcolorbox{boxA}{
    fontupper = \bf,
    boxrule = 1.5pt,
    colframe = black % frame color
}
\newtcolorbox{boxB}{
    fontupper = \bf\color{main}, % font color
    boxrule = 1.5pt,
    colframe = main,
    rounded corners,
    arc = 5pt   % corners roundness
}
\newtcolorbox{boxC}{
    colback = sub, % background color
    boxrule = 0pt  % no borders
}
\newtcolorbox{boxD}{
    colback = sub, 
    colframe = main, 
    boxrule = 0pt, 
    toprule = 3pt, % top rule weight
    bottomrule = 3pt % bottom rule weight
}
\newtcolorbox{boxE}{
    enhanced, % for a fancier setting,
    boxrule = 0pt, % clearing the default rule
    borderline = {0.75pt}{0pt}{main}, % outer line
    borderline = {0.75pt}{2pt}{sub} % inner line
}
\newtcolorbox{boxF}{
    colback = sub,
    enhanced,
    boxrule = 1.5pt, 
    colframe = white, % making the base for dash line
    borderline = {1.5pt}{0pt}{main, dashed} % add "dashed" for dashed line
}
\newtcolorbox{boxG}{
    enhanced,
    boxrule = 0pt,
    colback = sub,
    borderline west = {1pt}{0pt}{main}, 
    borderline west = {0.75pt}{2pt}{main}, 
    borderline east = {1pt}{0pt}{main}, 
    borderline east = {0.75pt}{2pt}{main}
}
\newtcolorbox{boxH}{
    colback = sub, 
    colframe = main, 
    boxrule = 0pt, 
    leftrule = 6pt % left rule weight
}
\newtcolorbox{boxI}{
    colback = sub, 
    colframe = main, 
    boxrule = 0pt, 
    toprule = 6pt % top rule weight
}
\newtcolorbox{boxJ}{
    sharpish corners, % better drop shadow
    colback = sub, 
    colframe = main, 
    boxrule = 0pt, 
    toprule = 4.5pt, % top rule weight
    enhanced,
    fuzzy shadow = {0pt}{-2pt}{-0.5pt}{0.5pt}{black!35} % {xshift}{yshift}{offset}{step}{options} 
}
\newtcolorbox{boxK}{
    sharpish corners, % better drop shadow
    boxrule = 0pt,
    toprule = 4.5pt, % top rule weight
    enhanced,
    fuzzy shadow = {0pt}{-2pt}{-0.5pt}{0.5pt}{black!35} % {xshift}{yshift}{offset}{step}{options} 
}
\newtcolorbox{boxL}{
    fontupper = \color{main},
    rounded corners,
    arc = 6pt,
    colback = sub, 
    colframe = main!50, 
    boxrule = 0pt, 
    bottomrule = 4.5pt 
}
\newtcolorbox{boxM}{
    fontupper = \color{white},
    rounded corners,
    arc = 6pt,
    colback = main!80, 
    colframe = main, 
    boxrule = 0pt, 
    bottomrule = 4.5pt,
    enhanced,
    fuzzy shadow = {0pt}{-3pt}{-0.5pt}{0.5pt}{black!35}
}
\begin{document}
{\LARGE Simple Box} % inspired by https://saruwakakun.com/html-css/reference/box
\begin{multicols}{2}
A. Just enclose with a solid line.
\begin{boxA}
It is just a frame surrounded by a slightly thick line. A simple monochrome design might be fine, but when you want a gorgeous look, it's a bit unsatisfactory.
\end{boxA}
B. Rounded Corner
\begin{boxB}
We changed the color and made the corners round. Change the text color, line color, and roundness of the corners as necessary.
\end{boxB}
C. Background Fill
\begin{boxC}
I tried to fill the background without using lines. Please change the background color to your liking.
\end{boxC}
D. Only Top and Bottom Line
\begin{boxD}
In addition to filling, I added thicker lines to the top and bottom. It is important to choose a color that is compatible with the lines and background.
\end{boxD}
E. Double Line Border
\begin{boxE}
A double-lined frame. You can create it by specifying multiple "borderline". The last argument in \texttt{borderline} is essentially work the same as \texttt{tikz} options.
\end{boxE}
F. Dashed Box
\begin{boxF}
I made the surrounding frame a broken line. Change the line thickness to your liking (just adjust the first argument in \texttt{borderline}).
\end{boxF}
G. Left and Right Double Line
\begin{boxG}
I tried adding a double line only to the left and right. We recommend changing the color according to your document design.
\end{boxG}
H. Thick Line Only to the left
\begin{boxH}
It is a sticky note box design. You may use it to write important points or supplementary notes in the article.
\end{boxH}
I. Thick line only on top
\begin{boxI}
This is the case when I added a line only to the top. The point is to put together the background and line/character colors in the same system color overall.
\end{boxI}
J. If You Cast a Shadow...
\begin{boxJ}
It looks like a little floating with a shadow. Although it has a mint color scheme, the background may be white or gray.
\end{boxJ}
K. Smart Design
\begin{boxK}
The box area is made visible by the shadow instead of the border. The top line may be placed on the left (replace border-top with border-left).
\end{boxK}
L. Thick Board Style
\begin{boxL}
The underline is slightly darker than the background to express the thickness. The neck is that it looks a little like a button.
\end{boxL}
M. Thick Board with Shadow
\begin{boxM}
I added a shadow to give it a more three-dimensional appearance. It's a little flashy, so it might be good to use it on the part you really want to stand out.
\end{boxM}
\end{multicols}
\end{document}