Change my background on all pages to Black with white or gray text

Trying to change background color to Black with white or gray text.

For example, I have some images on this page http://mosaiclegs.com/wp/studio/mediums/ with black, I would like for everything around it to be black as well. I have trying a custom CSS that I read in other forums but it didn’t work. Not sure what I am doing wrong.

.entry-header
{
background-color: black !important;
}

.entry-meta
{
background-color: black !important;
}

.entry-content
{
background-color: black !important;
}
#main-content{
background-color: #000000 !important;
}`

Anyone also know if vertex is slow or maybe its me but site seems to lag.