What exactly do you mean "you can't" and "you tried" - Did it actually save the css? Also, please make sure you are editing the THEME css stylesheet in admin, as flash Imagevue does not get any CSS from layout.phtml (which is strictly for the document that contains the SWF).
Also, I see you are using the embedded font that we provide in two of our themes. Unfortunately, this embedded font is a bitmap font and does not comply properly to CSS styles, because it is embedded in a fixed style and size. You would need to change font altogether if you want to play around with font sizes and styles ...