depends on which topics ( there are no threads in phpbb ) you are referring to.
open the file named navbar_header.html and/or navbar_footer in a good text editor.
make your changes as you wish, save the file, upload back to the server, purge the cache, refresh your browser. see if it works.
I am available for custom work on a donation basis. Please send me a PM with your needs.
Premium phpBB 3.2 Styles
OK, so what's the speed of dark?