This is all done in the CSS stylesheet. For the newlighttable, newdarktable, newsubjecttable, etc. elements, just include a font-style or font-weight element. Here's an example from one of the stylesheets I have available, where rather than bolding the new posts, they're italicized:
.newlighttable {<br /> background: #00194C;<br /> font-style: italic;<br /> color: #FFFFFF;
If you want the background color to not change, but just the text to change, simply make the background color the same as the non-new elements, and then use a font-style or font-weight entry for the text.