Hey there! :3
So you want the text in your signature to link to a thread? That isn't necessarily CSS, but just BB code. Here's the code you'd need to use:
Code:
[PLAIN][url=http://yoururlhere.com]your text here[/url][/PLAIN]
And if you have further questions, please ask those in our
CSS Help & Resources thread as well.
Hope this helps ya! :)