I have seem a lot of videos in which developers changing CSS on the fly in chrome. I tried the same thing but chrome dod not allow me to change the code. I can't write on the style sheet.

Is there any specific setting to do this? Kindly help.

EDIT: To edit the css, I right clock on an element, select inspect element. It will open the console. I select the id of the element and go to style.css in resource and try to change the css. It do not allow me to write there.