The question you have presented is difficult to answer for a variety of reasons.
The theme you are using is unknown. Some themes do have option panels that allow you to control font, font color, font size, etc. Some themes do not. Without knowing that theme you are using it would almost be impossible for anyone to offer you an answer.
In order to troubleshoot this yourself, a basic knowledge of CSS and HTML would be required. Many people use Chrome or Firefox and inspect individual elements on the page and modify them in memory to get the desired effect, then write it to their CSS files.
Perhaps someone would be more willing to take a stab at this if you were to post the version of WordPress you were using that name and version of the theme you were attempting to troubleshoot.
Bob
