Hi,
That must be a false positive, you can check the contents of ie.css in any text editor yourself. The location is themes/iconic-one-pro/css/ie.css in the current theme version.
It is referenced in header.php it is only meant for older ie versions.
.css files are harmless. It is the .js files you should worry about if malware scanner reports any.
It seems you are using a very old version of theme, please upgrade to the latest version of theme from the member area.
Upgrade guide and changelog available inside.