Here is an example you can start with:
#content .entry-content {
background: #fff;
}
#content .entry-content,
#content .entry-content h1,
#content .entry-content h2,
#content .entry-content h3 {
color: #000;
}
That example will turn the content area white and most content area text black for individual posts and pages in the Soundcheck theme. It does not affect category pages. Note that you may need to add other selectors to the list in the 2nd blog if you find that you add some text in the future that doesn't turn black. The example is based on your current content.
I couldn't find a way to turn the Likes post count text such as "One blogger likes this" black. This is because the Likes box is setup using an iframe and the source comes from a different domain. I don't see a way to update that specific line of text because of the way it has been setup.