You can set it yourself, just edit the `filetypes_extensions.conf` file (Menu->Tools->Configurations Files->filetype_extensions.conf` and add them to css. Don't forget to uncomment the line (and the [extensions] line near the top).
@elextr Thanks very much for that; I wasn't aware of that file existing before. I've added .scss and .less as extensions for CSS files.
I still think it would be a good idea for them to be there by default but I'm glad they're configurable by the user (as now I can do more obscure things which really have no need at all to be in by default, such as giving game data which uses a standard markup format but a non-standard extension the appropriate highlighting)