A Flash movie is inserted into an HTML page with the <object> tag (and until its use was deprecated, the <embed> tag). So, you could make a CSS style rule for all object elements on the page:

object { /* style properties */ }

Or, you could give your Flash object element a particular id or class and then select for those in the CSS. For example:

In the HTML:
<object class='FlashMovie' id='myMovie'>[Flash movie parameters]</object>

In the CSS:
object.FlashMovie { /* style properties */ }
or
object#myMovie { /* style properties */ }