Wauv, I figured it out my self after a looong time, but feels SO good :))
Maybe somebody can use it one day, so here it goes:
This can change font and size in the main menu
#header-menu > li {
font: normal 16px Georgia, serif;
margin: 0 0 0 5px;
}
This can change font and size in the slider
.slider_post h2 a,
.slider_post h2 a:visited {
font: normal 24px Georgia, serif;
}
This can change the headline on posts and also on frontpage
.post-title,
.post-title a {
font: 24px Georgia, serif;
}