Minor changes, fixes and additions

So I fixed a small issue, and then got sucked into making all kinds of tiny changes :D
The biggest one is probably that the logo is now horizontally centered, and there’s a bit of vertical space between “at your own pace” and the logo. (I suppose I’m now finally happy with its position, 6 months after I [“just stuffed it in the header”](https://github.com/deplicator/xkcdTime_atyourownpace/pull/140#issue-18194580) :D)