How is BBC changing colours based on temperature? CSS?

How is BBC changing the background colour of the day time temperature in the link below?

http://news.bbc.co.uk/weather/forecast/4276?&开发者_开发问答;search=gerrards%20cross&itemsPerPage=10&region=world&area=Gerrards%20Cross

Looks like the colour is based on the temperature. I've tried looking at there css files, but can't see any colours associated to those class names.

.core .temp-19-21, .extra-info .temp-19-21 {

in CSSMain.css

Just use a DOM inspector like Chrome's Inspect Element or Firebug.