Look at you page source and locate where the link is.
Note if there's any class tag in it. If there is, then you need to locate a tag with the same name in you CSS file and tweak there. If you won't find any class tag, there might be an id tag. If there is one, look in the header of your doc as it could be set there.
Now if that's not there as well see if there's something like style="" in your href tag and tweak it there.
If that fails, and your <a href> tag does not have any of the mentioned above -- make sure your page points to a correct css file, i.e. check the path to css.
It's not s bad as it sounds... Good luck.