I would like to have two middle columns in a grey background (constant) and on the top to overlay a square to highlight something in particular (just to enter at some point). An example of of what I would like follows - numbers are different from my MVE.
\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage{psfrag}
\usepackage{tikz}
\usetikzlibrary{intersections,patterns,pgfplots.fillbetween}
\usetikzlibrary{decorations.pathreplacing}
\usepackage{booktabs, dcolumn}
\usepackage{fontawesome}
\usetheme{CambridgeUS}
\usecolortheme{seahorse}
\usepackage{multirow}
\usepackage{hyperref}
\usepackage{pgfplots}
\usepgfplotslibrary{dateplot}
\usepackage{datetime}
\usepackage[scale=2]{ccicons}
\newdateformat{monthyeardate}{%
\monthname[\THEMONTH], \THEYEAR}
\usepackage{animate}
%\usepackage{colortbl}
\usepackage[beamer,customcolors]{hf-tikz}
\usetikzlibrary{calc}
\title{beamer}
% To set the highlighting boxes red.
\tikzset{%
hl/.style={
align=center,
set fill color=white,
set border color=red,
right offset={0.35,-0.2},
},
hl2/.style={
align=center,
set fill color=gray!10,
set border color = white,
right offset={0.35,-0.2},
},
}
\begin{document}
\maketitle
\begin{frame}
\begin{table}
\centering
\begin{tabular}{l*{6}{c}}
\toprule
& \multicolumn{2}{c}{x} & \tikzmarkin<1->[hl2]{c} \multicolumn{2}{c}{y} & \multicolumn{2}{c}{555} \\
\cmidrule(r){2-3} \cmidrule(r){4-5} \cmidrule(r){6-7} &\multicolumn{1}{c}{Probit}&\multicolumn{1}{c}{Heckman Prob}&\multicolumn{1}{c}{Probit}&\multicolumn{1}{c}{Heckman Prob}&\multicolumn{1}{c}{Probit}&\multicolumn{1}{c}{Heckman Prob}\\
& (1) & (2) & (3) & (4) & (5) &(6) \\
\midrule
wwwww & 75 & 85 & 850 & 980& 00 & 800\\
& 80 & 1400 & 250 & 70 & 199 & 60 \\
\multicolumn{7}{l}{cccccccccccccccc} \\
1 & \tikzmarkin<2>[hl]{bH5} 50 & 30 & 40 & 10 \tikzmarkend{c}\tikzmarkend{bH5} & 20 5\\
\end{tabular}
\end{table}
\end{frame}
\end{document}



\tikzmarkin{...}...\tikzmarkend{...}cannot be put over\cellcolororcolumncolor, consequently it is covered by them. your attempt seems to be request for newhf-tikzfeatures` – Zarko May 09 '19 at 13:11