Well, this is a bit of a quirk in column grid. It has existed for a while but I'll try and remember to fix it in the next release.
Try this somewhere in your stylesheet.
That is untested so I might have made typos.
It will only change the styling in the product listing not everywhere. What the css is saying is apply this styling to all the 'th' that are inside centerBoxContentsProducts.
It is well worth getting you head around a bit of css. Most styling is done that way. Another suggestion is to work out how to use the firebug or web developer tools. These are browser tools that let you see the id and class of elements on the page. If you know them then it is easy to write a css rule to target those elements.