\documentclass{report}
\usepackage[pagestyles, outermarks, newparttoc]{titlesec}
\usepackage[title]{appendix}
\usepackage{tocloft}
\newlength\mylength
\renewcommand\cftchappresnum{\chaptername~}
\renewcommand\cftchapaftersnum{}
\settowidth\mylength{\cftchappresnum\cftchapaftersnum\quad}
\addtolength\cftchapnumwidth{\mylength}
%\renewcommand\chaptername{Appendix}
\renewcommand{\appendixname}{Appendix}
\begin{document}
\tableofcontents
\chapter{Appendix testing}
\begin{appendices}
\chapter{Theory}
\section{subappendix}
\chapter{Theory}
\section{subappendix}
\end{appendices}
\end{document}
答案1
尝试对你的 MWE 进行这个小修改(对此表示感谢)。
% apptocnameprob.tex SE 589128
\documentclass{report}
\usepackage[pagestyles, outermarks, newparttoc]{titlesec}
\usepackage[title]{appendix}
\usepackage{tocloft}
\newlength\mylength
\renewcommand\cftchappresnum{\chaptername~}
\renewcommand\cftchapaftersnum{}
\settowidth\mylength{\cftchappresnum\cftchapaftersnum\quad}
\addtolength\cftchapnumwidth{\mylength}
%\renewcommand\chaptername{Appendix}
\renewcommand{\appendixname}{Appendix}
\begin{document}
\tableofcontents
\chapter{Appendix testing}
\begin{appendices}
%% At this point in the ToC change Chapter to Appendix
\addtocontents{toc}{%
\protect\renewcommand{\protect\chaptername}{Appendix}}
\chapter{Theory}
\section{subappendix}
\chapter{Theory}
\section{subappendix}
\end{appendices}
\end{document}