Theme CSS file missing

Hi,

I want to make some permanent changes to the CSS style of my web page and from what I've seen online I have to make the changes to the CSS file in the  "Myproject\Theme\Style" folder but in my project folder I can't find the Style folder within 'Theme'. Am I missing something here ?. I know I can make changes to the CSS file in the deployment folder but those changes are not permanent.

Any help would be very appreciated.

Thanks