Adding custom code

Yes, you can just enter a couple of times after the last curly bracket in your screenshot.

One thing that might help as you add more code is to include notes about what each is for (some isn't as obvious as your site title one!)

There are a few ways you can do this in CSS but here's the format I use:

// note about code here //
.selector {
  properties: xxxx;
}

// e.g. blog pagination spacing //
.selector {
  properties: xxxx;
}