You go into your Template styles and edit the CSS from there, i've been in there for edits before on Vbulletin, and it's not very well laid out, but pretty simple to fix. They are listed by category, otherwise there's a global CSS just like in MyBB. Find the hex value of that text which is most commonly what color values are stored in and Ctrl + F results that may be relevant to that CSS style depending on what class you find with something like Firebug in your webbrowser.