I am not sure what tutorial you mean as I don’t recall making one for Xtreme.
But on Xtreme, it is actually trivial.
First put a zero in Main options > HEader > Site Title / Tagline > Title Position X box AND the Tagline position X box.
Then add the rule below in Main Options > Header > Site Title / Tagline > Title tagline Area BG CSS+ box
{text-align:center;}
Remove anything else you did, and post back as is if there is still an issue