While looking at viewtopic.php?f=285&t=131263 I tried to find documentation about specifications for margins, paddings, font-sizes, ... for the "default KDE UI", but couldn't find anything. (eg something like http://www.google.com/design/spec/compo ... enus-specs)
Are these information anywhere?

Do these specifications depend just on the theme ?

We have guidelines for alignment (https://techbase.kde.org/Projects/Usabi ... /Alignment) and spacing/sizing (https://techbase.kde.org/Projects/Usabi ... /Placement). The latter is rather for minimum values since it depends on the actual theme how big margins are.