Color behind numbers in lstlisting

Question

Currently, my listings use this style :

\definecolor{lstgrey}{RGB}{217, 217, 217}
\definecolor{lstback}{RGB}{246, 246, 246}
\newlength{\numberlengthnormal}
\setlength{\numberlengthnormal}{\widthof{\fontfamily{cmtt}\selectfont 999}}
\setlength{\numberlengthnormal}{0.25\numberlengthnormal}
\lstnewenvironment{lstcpp}
{\lstset{%
    language = C++,
    tabsize=2, 
    basicstyle=\fontfamily{cmtt}\selectfont,
    keywordstyle=\color{blue},
    commentstyle=\color{green},
    morekeywords={constexpr, kind, parameter, allocatable},
    showstringspaces = false,
    backgroundcolor=\color{lstback},
    numbers = left,
    numbersep = 2\numberlengthnormal, 
    xleftmargin = 6\numberlengthnormal,
}}%

How to color the background of numbers as in the following example ?

Answer 1

You were almost there.

\documentclass{article}
\usepackage{xcolor,listings,calc}
\definecolor{lstgrey}{RGB}{217, 217, 217}
\definecolor{lstback}{RGB}{246, 246, 246}
\newlength{\numberlengthnormal}
\setlength{\numberlengthnormal}{\widthof{\fontfamily{cmtt}\selectfont 999}}
\lstnewenvironment{lstcpp}[1][]
  {\lstset{%
    language = C++,
    tabsize=2, 
    basicstyle=\fontfamily{cmtt}\selectfont,
    numberstyle=\darknumbers,
    keywordstyle=\color{blue},
    commentstyle=\color{green},
    morekeywords={constexpr, kind, parameter, allocatable},
    showstringspaces = false,
    backgroundcolor=\color{lstback},
    numbers = left,
    numbersep = 0pt,
    xleftmargin = \numberlengthnormal,
    #1,
  }}{}

\newcommand{\darknumbers}[1]{%
  \begingroup\fboxsep=0pt
  \colorbox{lstgrey}{\makebox[\numberlengthnormal]{\strut#1}}%
  \endgroup
}

\begin{document}

\begin{lstcpp}
#include <iostream>
#include <cmath>
\end{lstcpp}

\end{document}