lib/texinputs/isabelle.sty

 \def\isacharbackquoteclose{\isatext{\raise.3ex\hbox{$\scriptscriptstyle\rangle$}}}%
 \def\isacharverbatimopen{\isamath{\langle\!\langle}}%
 \def\isacharverbatimclose{\isamath{\rangle\!\rangle}}%
 }
 
-\newcommand{\isabellestyleitunderscore}{%
+\newcommand{\isabellestyleliteral}{%
 \isabellestyleit%
 \def\isacharunderscore{\_}%
 \def\isacharunderscorekeyword{\_}%
+\chardef\isacharbackquoteopen=`\`%
+\chardef\isacharbackquoteclose=`\`%
 }
 
 \newcommand{\isabellestylesl}{%
 \isabellestyleit%
 \def\isastyle{\small\sl}%