There’s a CSS rule being output in the <head> of that document that is defining the color, after your stylesheet is included (thus overriding it). Take a look at the options for the theme, and see if there’s a way to set the color through the admin area.
If you want to override JUST the description, add !important to the color rule in your css file. It’s an unfortunate solution, since !important rules can cause all kinds of havoc during maintenance phases… but the other option is to dig into the theme files and try to change the options settings to two color settings, or change the output in the theme header so it only defines a color for the title, instead of both the title and description.