css3 - css what is the "at" property? -

css3 - css what is the "at" property? -

i found css file on net starts

@sr: #418a95; @srlight: #418a95 + #222; @srdark: #418a95 - #222; @darkgrey:#6b6b6b; @offwhite:#f6f3f7; @bg:#f6f3f7 - #111; @darkbg:#f6f3f7 - #222;

and in code, this:

background:@sr;

though ide, phpstorm, tells me not right css style.

what please?

edit

this website http://codepen.io/alimudassar111/pen/idyiq

it's less, css preprocessor, , @ sign marks variable.

css css3 phpstorm