You can add a block to the header using one of the layout files.
<default>
<reference name="head"><!-- this means you are adding to the head block -->
<block type="core/template" name="custom_styles" as="custom_styles" template="custom/styles.phtml" /><!-- this is a general block with a custom template -->
</reference>
</default>
and in app/design/frontend/{interface}/{theme}/template/custom/styles.phtml
add this:
<style>
/* your styles here*/
</style>
You can even add some custom js in the same file. After the styles above add:
<script type="text/javascript">
//your js here
</script>
Clear the cache and give it a try.