How to color a cell of table completely?

Question

I want to color the top row of my table. However, when I used the \rowcolor command, there are white spaces appearing in the second and third columns (cell is not filling completely with the color). How can I solve this? I am facing this problem over multiple tables. Is it because of a clash between some packages or something else?

\documentclass[11pt,a4paper,twoside,openrights,hidelinks]{book}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage{multirow}
\usepackage{amsmath,amssymb}
\usepackage{setspace}
\usepackage{makecell}
\usepackage{tabu}
\usepackage{threeparttablex}
\usepackage{hhline}
\usepackage[RGYK]{xcolor}
\usepackage{colortbl}
\begin{document}
\begin{table}[t!]
    \renewcommand{\arraystretch}{1.3} 
    \centering
    \scriptsize
    \caption{Feeder energy losses for 10 days before and after phase switching of the loads for both phase switching algorithms.}
    \begin{threeparttable}
        \centering
        \begin{tabular}{|c|c|c|c|c|c|c|}
            \hline
            \rowcolor{gray!50}      \textbf{Feeder} & \multicolumn{3}{c|}{\thead{\thead{\textbf{Losses for} \textbf{Passive Network} \\ \textbf{(Scenario S1)}} \\ $[\textbf{kWh}]$}}& \multicolumn{3}{c|}{\thead{\thead{\textbf{Losses for} \textbf{Active Network} \\ \textbf{(Scenario S2)}} \\ $[\textbf{kWh}]$}}\\
            \hline
            & \textbf{BC} & \textbf{PSA-1} & \textbf{PSA-2} & \textbf{BC} & \textbf{PSA-1}  & \textbf{PSA-2}  \\ \hline
            \textit{Feeder 1}        &38.3  &30.7   (-19.8\%) &37.0  (-3.4\%)  &38.3  &30.7  (-19.8\%) &37.0  (-3.4\%)\\ \hline 
            \textit{Feeder 2}        &73.2  &53.2   (-27.5\%) &65.0  (-11.2\%) &71.1  &49.7  (-30.1\%) &61.1  (-14.1\%)\\ \hline
            \textit{Feeder} 3        &493.6 &278.1  (-43.6\%) &339.1 (-31.3\%) &487.0 &266.5 (-45.3\%) &332.0 (-31.8\%)\\ \hline 
            \textit{Network}         &605.1 &362.0  (-40.2\%) &441.1 (-27.1\%) &596.4 &346.9 (-41.8\%) &430.1 (-27.8\%)\\ \hline
            %       \end{tabularx}
        \end{tabular}
    \end{threeparttable}
    \begin{tablenotes}\footnotesize
        \item BC: Base Case 
    \end{tablenotes}
    \label{chp4_Table1}
\end{table}
\end{document}

Answer 1

You can play with the overhang optional arguments of \rowcolor (this is by trial and error and depends on the font size). I took the liberty to simplify your code: there's no need for nested \theads, and colortbl is automatically loaded by the table option of xcolor. Also, reducing the value of \tabcolsep, I could increase the font size (for better readability) without overflowing into the margin.

\documentclass[11pt,a4paper,twoside,openrights,hidelinks, english]{book}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{babel}
\usepackage{multirow}
\usepackage{amsmath,amssymb}
\usepackage{setspace}
\usepackage{makecell}
\usepackage{tabu}
\usepackage{threeparttablex}
\usepackage{hhline}
\usepackage[RGYK, table]{xcolor}

\begin{document}

\begin{table}[t!]
    \renewcommand{\arraystretch}{1.3}
    \renewcommand{\theadfont}{\bfseries}
    \setlength{\tabcolsep}{3.25pt}
    \centering
    \footnotesize
    \begin{threeparttable}
    \caption{Feeder energy losses for 10 days before and after phase switching of the loads for both phase switching algorithms.}
        \begin{tabular}{|*{7}{c|}}
            \hline
            \rowcolor{gray!50}[\tabcolsep][2.5em]\textbf{Feeder} & \multicolumn{3}{c|}{\thead{Losses for Passive Network \\ (Scenario S1) \\[1ex] [kWh]}}& \multicolumn{3}{c|}{\thead{Losses for Active Network \\ (Scenario S2) \\[1ex] [kWh]}}
            \\ \hline
            & \textbf{BC} & \textbf{PSA-1} & \textbf{PSA-2} & \textbf{BC} & \textbf{PSA-1} & \textbf{PSA-2} \\ \hline
            \textit{Feeder 1} &38.3 &30.7 (-19.8\%) &37.0 (-3.4\%) &38.3 &30.7 (-19.8\%) &37.0 (-3.4\%)\\ \hline
            \textit{Feeder 2} &73.2 &53.2 (-27.5\%) &65.0 (-11.2\%) &71.1 &49.7 (-30.1\%) &61.1 (-14.1\%)\\ \hline
            \textit{Feeder} 3 &493.6 &278.1 (-43.6\%) &339.1 (-31.3\%) &487.0 &266.5 (-45.3\%) &332.0 (-31.8\%)\\ \hline
            \textit{Network} &605.1 &362.0 (-40.2\%) &441.1 (-27.1\%) &596.4 &346.9 (-41.8\%) &430.1 (-27.8\%)\\ \hline
        \end{tabular}
    \end{threeparttable}
    \begin{tablenotes}\footnotesize
        \item BC: Base Case
    \end{tablenotes}
    \label{chp4_Table1}
\end{table}

\end{document} 

Answer 2

\thead doesn't occupy complete column width, consequently you obtain white space. Instead using it I suggest to define new command:

\newcommand\mcccc[1]{\multicolumn{3}{>{\bfseries}c|}{%
                \begin{tabular}{ @{} >{\bfseries}c @{}} #1 \end{tabular}}}

which will eliminate white space:

\documentclass[11pt,a4paper,twoside,openrights,hidelinks]{book}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage{multirow}
\usepackage{amsmath,amssymb}
\usepackage{setspace}
\usepackage{makecell}
\renewcommand\theadfont{\normalsize\bfseries}           %  <---
\newcolumntype{C}[1]{>{\centering\arraybackslash}p{#1}} % <---

\newcommand\mcccc[1]{\multicolumn{3}{>{\bfseries}c|}{%
                \begin{tabular}{ @{} >{\bfseries}c @{}} #1 \end{tabular}}}
\usepackage{tabu}  % not recomended, it is buggy
\usepackage{threeparttablex}
\usepackage{hhline}
\usepackage[RGYK, table]{xcolor}

\begin{document}
\begin{table}[t!]
    \renewcommand{\arraystretch}{1.3}
    \centering
    \scriptsize
    \caption{Feeder energy losses for 10 days before and after phase switching of the loads for both phase switching algorithms.}
    \begin{threeparttable}
        \centering
        \begin{tabular}{|*{7}{c|}}
            \hline
            \rowcolor{gray!30} 
        \textbf{Feeder} 
                & \mcccc{Losses for Passive Network\\
                        (Scenario S1)\\
                        {[kWh]} }
                    & \mcccc{Losses for Active Network\\
                            (Scenario S2)\\
                            {[kWh]}}                  \\
            \hline
            & \textbf{BC} & \textbf{PSA-1} & \textbf{PSA-2} & \textbf{BC} & \textbf{PSA-1}  & \textbf{PSA-2}  \\ \hline
            \textit{Feeder 1}        &38.3  &30.7   (-19.8\%) &37.0  (-3.4\%)  &38.3  &30.7  (-19.8\%) &37.0  (-3.4\%)\\ \hline
            \textit{Feeder 2}        &73.2  &53.2   (-27.5\%) &65.0  (-11.2\%) &71.1  &49.7  (-30.1\%) &61.1  (-14.1\%)\\ \hline
            \textit{Feeder} 3        &493.6 &278.1  (-43.6\%) &339.1 (-31.3\%) &487.0 &266.5 (-45.3\%) &332.0 (-31.8\%)\\ \hline
            \textit{Network}         &605.1 &362.0  (-40.2\%) &441.1 (-27.1\%) &596.4 &346.9 (-41.8\%) &430.1 (-27.8\%)\\ \hline
            %       \end{tabularx}
        \end{tabular}
    \end{threeparttable}
    \begin{tablenotes}\footnotesize
        \item BC: Base Case
    \end{tablenotes}
    \label{chp4_Table1}
\end{table}
\end{document}

Answer 3

If you use {NiceTabular} of nicematrix with its key color-inside, you have directly the expected output (I have also use the key hvlines) to draw all the rules.

\documentclass[11pt,a4paper,twoside,openrights,hidelinks]{book}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[english]{babel}
\usepackage{amsmath,amssymb}
\usepackage{setspace}
\usepackage{makecell}
\usepackage{threeparttablex}
\usepackage{xcolor}
\usepackage{nicematrix}


\begin{document}
\begin{table}[t!]
\renewcommand{\arraystretch}{1.3} 
\centering
\scriptsize
\caption{Feeder energy losses for 10 days before and after phase switching of the loads for both phase switching algorithms.}
\begin{threeparttable}
\centering
\begin{NiceTabular}{ccccccc}[color-inside,hvlines]
    \rowcolor{gray!50}      \textbf{Feeder} & \multicolumn{3}{c}{\thead{\thead{\textbf{Losses for} \textbf{Passive Network} \\ \textbf{(Scenario S1)}} \\ $[\textbf{kWh}]$}}& \multicolumn{3}{c}{\thead{\thead{\textbf{Losses for} \textbf{Active Network} \\ \textbf{(Scenario S2)}} \\ $[\textbf{kWh}]$}}\\
    
    & \textbf{BC} & \textbf{PSA-1} & \textbf{PSA-2} & \textbf{BC} & \textbf{PSA-1}  & \textbf{PSA-2}  \\ 
    \textit{Feeder 1}        &38.3  &30.7   (-19.8\%) &37.0  (-3.4\%)  &38.3  &30.7  (-19.8\%) &37.0  (-3.4\%)\\  
    \textit{Feeder 2}        &73.2  &53.2   (-27.5\%) &65.0  (-11.2\%) &71.1  &49.7  (-30.1\%) &61.1  (-14.1\%)\\ 
    \textit{Feeder} 3        &493.6 &278.1  (-43.6\%) &339.1 (-31.3\%) &487.0 &266.5 (-45.3\%) &332.0 (-31.8\%)\\  
    \textit{Network}         &605.1 &362.0  (-40.2\%) &441.1 (-27.1\%) &596.4 &346.9 (-41.8\%) &430.1 (-27.8\%)\\ 
\end{NiceTabular}
\end{threeparttable}
\begin{tablenotes}\footnotesize
    \item BC: Base Case 
\end{tablenotes}
\label{chp4_Table1}
\end{table}
\end{document}