I tried to make the background of Nishita really blackblack with the following code:
#background
{
color: black;
}
The first part is a selector. You would only want to use "#background" as a selector if there were an HTML element on the site using id="background". So let's try using "#header" instead as an example. Also, the the "color" property is for changing font color, you want to use the "background" property to change the background. Here is a very basic example for changing just the header background to black so you can see how it works:
#header,
#header-inner {
background: black;
}
To change the background color for more things, you need to add a selector to that list for anything that's currently gray that you can find. Here is an example which also removes the borders for those elements as well:
#header,
#header-inner,
h2.photo-title,
h2.photo-title a,
.content-title,
.photo,
.photo-inner,
.photo-meta,
.photo-meta-inner,
.navigate a,
.navigate-single div,
.navigate a span,
.navigate-single a {
background: black;
border: 0;
}
Note that I didn't adjust the sidebar but you could add that too. I also didn't account for all post formats, and you may find others that you want to add to the list as you continue to update your blog.
Looks like you might be confusing selectors and properties a little. Give this tutorial a try to help with that:
http://www.htmldog.com/guides/cssbeginner/