Currently in my css files using the keyboard shortcut for a comment it will apply a comment with //.
However I would like to use a comment style like so /* comment */
Are you able to change this in PhpStorm/WebStorm?
// is not valid as comment symbol in plain CSS files. The IDE uses correct /* */ style here.
If the IDE actually uses that for comments in CSS files then check what CSS Dialect is set in Settings/Preferences | Languages & Frameworks | Stylesheets | Dialects. It sounds like it may set it to PostCSS (where // is valid).
Another possibility: *.css files are associated with some other File Type (e.g. already mentioned PostCSS or perhaps Less/Sass).
In any case: you may force /* */ for comments even for PostCSS: just use the Block Comment shortcut instead of just Comment. You will have to make a selection first though...
Code | Comment with Block Comment (Ctrl + Shift + / using most keymaps).