How can I modified css?

I would like to make my owen css to embed.ly plugin, becouse the text is unreadble and must be change color and font weight.
But somehow my css is not working.
I tryed this:
.description {
color: white !important;
}
But dosen’t work, becouse the text color is:
#464646;
Please help me change text colors.
The header information color also is must be white!

https://ww.wp.xz.cn/plugins/embedly/