In theory yes you can, but the practicality is somewhat different
here is a quote from blooberry.com research into the support of CSS for frames:
* CSS: Before IE v5.5 Beta 1, FRAMEs and IFRAMEs could not use the 'z-index' property (they would always be on the "top" of the rendering surface and nothing could be placed "above" them) and could not be transparent (such that content from positioned content beneath would "shine through".) Beginning in IE v5.5 Beta 1, these capabilities now exist.
* Netscape 6 and CSS: Even though support for the %Core% attributes is listed here, in direct testing it seemed that VERY few CSS properties could successfully be applied to this element.
if it's any help I found that using the "normal" HTML properties to set the main settings on the <frameset> element then it's easier to customise the individual <frame> elements...as frameset didn't want to accept border: 0px; and I don't think there's an equivalent of the "framespacing" attribute
e.g. this example shows the same effect being achieved on the two frames one the usual way, the other the external way CSS: