Hiding the sidebar on my first passage ?

Hello!

I am currently working on my first Twine and it's really fun. I just need some help with something.
I have created some sort of "splashscreen" for my game, composed of a welcoming sign, and a continue button. The button is centered, but not the big sign. I can't get the button to stay centered above the big sign...

Check it out here : http://josselin.colletta.org/clamecy/index.html

And I think it's because the sidebar is pushing the big sign sideways.
I was wondering if there was any way to hide the sidebar for this very first passage, and I can't find the answer online.

I am using Harlowe on Twine 2.

Thank you for your help,
Josselin