[Theme: Blissfulblog] Post-title centre

Hi everybody!

I spent hours today trying to center my post-titles in my CSS for the blissfulblog theme, but whatever I modify (entry title, h1, etc) nothing moves and nothing changes.
Can someone help me with that? Usually it’s something easy for me but here, I really don’t understand. Is there something hidden that I did not find?

Many thanks (and please forgive my english, I’m french).