Hi, after quite long playing with achieving layout I decide to write down what I found.
Short Version -------------
To get nice box centered at middle of screen, but content in box left aligned use HBox( HStretch, VBox( VStretch, Left(<content>), VStretch ), HStretch )
Long Version ------------
It start with feature to adapt theme and our designed team found some strange alignment that they want to change in dialog layout. So I start with trivial idea to have something like `Center(VBox(Left(*content)))` This sadly does not work as it in the end create box over whole place and everything is ugly on left side. which looks strange ( see screenshots in ). So next attempt with a bit of help is MarginBox. It looks fine and is shrinkable, so even on very small screen in ncurses looks fine, but in qt it is hard to pick good sizes as with different resolutions it is never on center for all resolutions. So good to make some spacing, but not to align box. The proper solution is to use that stretches around that create spacing around box that is proportional and content in Box then can left alignment with proper position.
Pro tip: If you do not want center but e.g. in one third, it is possible to use Weight to stretch. See documentation at 
 https://github.com/yast/yast-registration/pull/519  https://yast-ui-bindings.surge.sh/classYWeight.html