As benmarte said, the chunk you use for the [tt]&cssTpl[/tt] parameter may contain CSS rules wrapped by [tt]<style type="text/css>[/tt] and [tt]</style>[/tt].
There is another possibility: that chunk could contain the URL of a CSS file containing your rules, f.i. [tt]assets/css/mymenu.css[/tt] .
This will be output in the source code just before the closing </head> in this form:
<link rel="stylesheet" href="assets/css/mymenu.css" type="text/css" />
This wrapping by [tt]<link rel="stylesheet" href="[/tt] and [tt]" type="text/css" />[/tt] will always be done if the chunk for [tt]&cssTpl[/tt] does neither contain [tt]<style[/tt] nor [tt]<link[/tt] - that’s what happened to the chunk used by Lim0n.
Otherwise the unmodified chunk will be inserted in the <head> section - that’s the case for benmarte’s chunk.
(BTW: I’d always prefer the use of an external css file.)
Edit:
If there are more than one Wayfinder calls on a page, the [tt]&cssTpl[/tt] parameter in just one of it will effect the other menus too, because of its resulting in a CSS definition in the <head> section which rules the whole page.