\usepackage{etoolbox}
\makeatletter
\providecommand{\subtitle}[1]{% add subtitle to \maketitle
  \apptocmd{\@title}{\par {\large #1 \par}}{}{}
}
\makeatother

\usepackage{graphicx}
\graphicspath{{img/}}
\usepackage{pdfpages}

\renewcommand{\labelitemi}{$\circ$}
% labelitemii, iii, iv, ... kan ook

\usepackage{tcolorbox}