If I understand you right, you want to change the date/time box that appears when you click on a single post.
There is an article in our documentation site called Finding Your CSS Styles and it will help you locate the styles within your web page that need changing.
Using those techniques, I found that your date and time information is in a section called the "post meta data" and it looks like this:
<p class="postmetadata alt"><small>
Pay attention to the first word "postmetadata" or the word "small" and look in your
style.css file to find that word. There you will find all the information you need to style this section. The keyword you are looking for in the code is "color". That is where you put your color code, replacing what was there.
There is also an article on Styling Your Post Meta Data Section in the Codex that takes you step by step through the process of changing this area.