In your CSS file, that particular form of the color specification is a straight HTML color specification, in hexadecimal numbers. The 1st 2 characters are a percent of RED, the next 2 characters are a percent of GREEN and the next 2 characters are a percent of BLUE. The thing is that 00, 33 and 99 are HEX numbers.
#FF0000 = FF=256 or 100% of RED, 0% green , 0% blue
#00FF00 = 0% RED, 100% GREEN, 0% BLUE
#003399 = 0% RED, approx 20% GREEN, approx 60% blue or sort of dark blue