Changing the QED sign to tombstone

ctufit-thesis.cls

@@ -443,7 +443,6 @@
   \thm@postskip\thm@preskip
 }
 \def\@endtheorem{\endtrivlist}%\@endpefalse
-\renewcommand\qedsymbol{\textcolor{decoration}{\ensuremath{\blacktriangleleft}}}
 \renewenvironment{proof}[1][\proofname]{\par
   \pushQED{\qed}%
   \normalfont \topsep6\p@\@plus6\p@\relax