i specified color attribute

I specified color attributes for the blackground and foreground colors in my epub file and then added it to Calibre. i converted the epub file to a mobi file and then used the Kindle for PC to read the book. The color attributes did not change the colors. I set the color attribute in the body tag of my CSS style file.

I can not figure out why the colors did not change. Anybody know why?

Thanks
Ronaldl