Why can't I load miniframes outer theme after Szeged theme?

Question

Making some tests to answer omit delete second bar under the navigation bar in szeged theme I found that

\documentclass{beamer}

\usetheme{Szeged}
\useoutertheme[subsection=false]{miniframes}

\begin{document}
\frame{Hello} 
\end{document}

finishes with ! LaTeX Error: Option clash for package beamerouterthememiniframes. and I would like to understand why is it.

Szeged already load miniframes with another option but I thought that was always possible to load other themes in order to combine them.

Answer 1

You could pass the option explicitely to the package, which you could do even before \documentclass. Note, we use the full theme package name here:

\PassOptionsToPackage{subsection=false}{beamerouterthememiniframes}
\documentclass{beamer}
\usetheme{Szeged} % implicitely loads miniframes
\begin{document}
\section{A section}
\subsection{A subsection}
\frame{Hello} 
\end{document}

Szeged loads miniframes just with a footline option:

\useoutertheme[footline=institutetitle]{miniframes}

This way, miniframes will work also with your option and will not show the subsection line in the header.