Section title with background color and line breaks in KOMA

Question

I want my subsection title to have a colored background with support for line breaks. There are a few questions about spanning that background to the margins - that's not what I want to achieve, however.

After some research I found that the soul package comes pretty close to what I need, but I can't, unfortunately, seem to use its \hl command in KOMA's \addtokomafont.

Here's an MWE

\documentclass[a5paper,pagesize]{scrbook}
\usepackage{xcolor,soul}
\sethlcolor{black}
\addtokomafont{subsection}{\hl\mdseries\color{red}}

\begin{document}
\subsection{A very peculiar title which will hopefully break into the next line}
\end{document}

Answer 1

You can redefine \sectionlinesformat (needs KOMA-Script Version 3.19 or newer) and \subsectionformat.

\documentclass[a5paper]{scrbook}[2015/10/03]
\usepackage{xcolor,soul}
\sethlcolor{black}

\makeatletter
\renewcommand\sectionlinesformat[4]{%
  \ifstr{#1}{subsection}% only subsections should be changed
    {\@hangfrom{\hskip#2 #3}{\hl{#4}}}% subsection level
    {\@hangfrom{\hskip#2 #3}{#4}}% other section levels
}
\makeatother
\addtokomafont{subsection}{\mdseries\color{red}}
\renewcommand\subsectionformat{\hl{\thesubsection\autodot}\enskip}

\begin{document}
\subsection{A very peculiar title which will hopefully break into the next line}
\end{document}

Note that \hl has some limitations. See the documentation of package soul.

---

If you really want to use \MakeUppercase for the subsections you have to change the redefinition of \sectionlinesformat to

\renewcommand\sectionlinesformat[4]{%
  \ifstr{#1}{subsection}% only subsections should be changed
    {\@hangfrom{\hskip#2 #3}{\hl{\MakeUppercase{#4}}}}% subsection level
    {\@hangfrom{\hskip#2 #3}{#4}}% other section levels
}

Result: