PassOptionsToPackage{svgnames,table,dvipsnames}{xcolor}
documentclass[a4paper,10pt,oneside,DIV=9]{scrartcl}

% Packages
usepackage{scrlayer-scrpage}
usepackage{xcolor} % showframe loads this
usepackage[normalem]{ulem}
usepackage{hyperref}

% demo only
usepackage{lipsum}
usepackage{mwe}

% theme colours
definecolor{MyThemeColour}{cmyk}{0.10,0.10,1.00,0} % personal yellow

% KOMA and general setup
renewcommand*{sectionformat}{%
  llap{thesectionautodotenskip}%
}
renewcommand*{subsectionformat}{%
  llap{thesubsectionautodotenskip}%
}
renewcommand*{subsubsectionformat}{%
  llap{thesubsubsectionautodotenskip}%
}

% underline section headings
makeatletter
renewcommand{sectionlinesformat}[4]{%
    @hangfrom{hskip #2expandafterheadulineexpandafter{#3}}%
    {#4}%
}
makeatother
newcommandheaduline{%
    bgroupmarkoverwith{textcolor{MyThemeColour}{rule[-0.75ex]{1pt}{1pt}}}ULon% 2pt 2pt
}
AtBeginDocument{renewcommandSectionformat[2]{headuline{#1}}}

% recalc the text block after all the font loading
KOMAoptions{DIV=last}

begin{document}
section{Lipsum}
blinddocument
end{document}