Do something about "magic numbers" in CSS examples

[bertfrees]

See for example the Simple box with blank lines example:

.box {
   border-style: solid;
   border-width: .45rem 0;
   margin: 1.22rem 0 1.55rem 0;
   padding: .33rem 0 0 0;
}

Either these numbers need an explanation (how to obtain the numbers?), or we need to recommend another approach that doesn't need the numbers (e.g. we could further develop the "border image" approach).

[bertfrees]

See #261