[Tex/LaTex] outline package, how to change enumeration style

Question

I'm working with the outlines package. I want to change the 2nd level enumeration style from small letters ( a b c ) to small roman letters ( i),ii),iii)… ). In other words, I want to exchange the 2nd and 3rd level enumeration styles. I tried reading the documentation of the package but I didn't quite understand if I have to write my own code or just apply what's suggested in the doc. I think it may involve
\renewcommand{\outlineii}{style} but what do I put in place of style, roman? I'd like this change to affect all 2nd-level items.

\documentclass[a4paper, 12pt]{scrartcl}

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[italian]{babel}

\usepackage{amssymb, amsmath, amsfonts} 
\usepackage{outlines}

\begin{document}

\section{First section} 
\subsection{First subsec} 
\begin{outline}[enumerate]

\1 Calculate $ E_{th} $.
\2 Write $f= f_{electr} + f_{elast}$:
\2 Apply Euler-Lagrange method 
\2 Calc each ``piece'' of the 2nd degree diff eq
  \3 first this
  \3 then the double derivative  
\2 Simplify \dots

\end{outline}
\end{document} 

Answer 1

outlines takes the styles from enumerate labels. You can change these with the help of enumitem package.

\documentclass[a4paper, 12pt]{scrartcl}

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[italian]{babel}

\usepackage{amssymb, amsmath, amsfonts}
\usepackage{outlines}

\usepackage{enumitem}
\setlist[enumerate,2]{label=\roman*)}
\setlist[enumerate,3]{label=\alph*)}

\begin{document}

\section{First section}
\subsection{First subsec}
\begin{outline}[enumerate]

\1 Calculate $ E_{th} $.
\2 Write $f= f_{electr} + f_{elast}$:
\2 Apply Euler-Lagrange method
\2 Calc each ``piece'' of the 2nd degree diff eq
  \3 first this
  \3 then the double derivative
\2 Simplify \dots

\end{outline}
\end{document}