Use lmodern

cel-thesis.cls

@@ -56,6 +56,7 @@
 % Gerneral %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 \usepackage[T1]{fontenc}  % encoding for target files.
 \usepackage[utf8]{inputenc} % UTF-8 encoding for source files
+\usepackage{lmodern}
 \usepackage[printonlyused]{acronym} % use this package for abbreviations!
 \def\setlanguage{
     \usepackage[\theThesisLanguage]{babel}