Hi Daniel,
You should be able to modify the text of the category headings by opening the ProductListSingleColumn.ascx and adding a class to the following cell:
<td colspan="4"><asp:Label runat="server" ID="lblCatDescInline" /></td>
Then add the new class to your module.css file and style to your liking.
Please let us know if the above solves this issue.
Thanks,