I'm not really shure, why it works that way, I just changed it and now I can click the menue open and you can close them.
body {
font-size: 10px;
font-family: Verdana, sans-serif; (HERE I CHANGED THE FONT)
background: #E8E8E8;
line-height: 1.3em;
/* font-weight: 300; */
}
/* active exhibit title parts */
li.active a:link, li a.active, li.active a:hover, li a.active, li.active a:active, li a.active,
li.active a:visited, li a.active, li span.active, #index ul.section li.active a:link,
#index ul.section li.active a:hover, #index ul.section li.active a:active, #index ul.section li.active a:visited
{ font-weight: normal; } (HERE I CHANGED THE FONT-WEIGHT FROM BOLD TO NORMAL )