DOCBOOK-APPS: internal margin in <screen> and other blocks

I followed hints from
http://www.dpawson.co.uk/docbook/styling/fo.html#d2024e464 and created box
around monospaced blocks. But how to increase distance between frame and
content of it ?

Also I often have this blocks as parts of examples. How can I create frame
around examples (and only examples) without using table? (or rather table in
docbook source but it can be table in resulted fo).

I use xsltproc -> 1.58 -> FOP.

ABX