updates of preface

structure.tex

@@ -628,7 +628,7 @@ addtohook={%
 \definecolor{ublue}{rgb}{0.152,0.250,0.545}
 \definecolor{ugreen}{rgb}{0,0.5,0}
 \definecolor{lgreen}{rgb}{0.9,1,0.8}
-\definecolor{xtgreen1}{rgb}{0.824,0.898,0.8}
+\definecolor{amber}{rgb}{1.0, 0.75, 0.0}
 \definecolor{xtgreen}{rgb}{0.914,0.945,0.902}
 \definecolor{lightgray}{gray}{0.85}