Unable to center gallery

I’ve been trying to use Fotorama (gallery) with Decode, but I don’t know how to center it. (It works perfectly on iphone, but ends up to the left on my computer – https://tomasjakobsson.com) I thought I could just paste the suggestions above into the box: customize/other options/custom css – but that changes nothing. Don’t know anything really about css, so I would be grateful for any help – it’s a great looking theme. What am I doing wrong?