Sliding sidebar icon

Hi,

I am using your Writing Theme (which is brilliant by the way) on my website http://www.duncanvandervelden.nl.

Now I would like to change the button appearance for the sliding sidebar, because I’d like for it to be a little more ‘eye-catching’. With the current button my visitors often don’t even realise that it is a button showing a hidden sidebar.

How do I do that?