OIC, then it's not a dup of #5672, since in this ticket BorderContainer is a direct child of TabContainer.
The first patch is probably OK... the second and third patch will work in the common case but will fail sometimes because they assume that the document has only one theme which is indicated on the <body> node. In other words, a splitter as a child of <body> may have a different width than the splitter as a child of a BorderContainer in some random place, because different CSS rules could be affecting it.