How to overwrite CSS in a child theme or custom CSS file. Here I want to change the background colour and colour of an element with the class of "my_cool_css_class".

custom.css

/* Parent CSS file*/
.my_cool_css_class {background:#fff;border-radius:5px;box-shadow:1px 1px 2px #000;color:#ff2361;padding:0px 10px;line-height:25px;height:25px;width:100%;}

/* This is WRONG. Very WRONG. */
/* Child Theme or Custom CSS file */
.my_cool_css_class {background:#2d2d2d;border-radius:5px;box-shadow:1px 1px 2px #000;color:#fff;padding:0px 10px;line-height:25px;height:25px;width:100%;}

/* This is RIGHT. Very RIGHT. */
/* Child Theme or Custom CSS file */
.my_cool_css_class {background:#2d2d2d;color:#fff;}