Custom css script doesn’t seem to work

I have added a custom css script that should change the skin "contentBottom" so that the background is a slightly transparant white and the text color is grey rgba(100,100,100). This is my custom css:

.skin.contentbottom .layer-content {
background: rgba(255, 255, 255, 0.9) none repeat scroll 0 0;
color: rgb(100, 100, 100);
}

It does not seem to work however. Anyone can help me to fix this?
Thanks,
Bart