As @tripod mentioned, there is a rather simple workaround, however I disliked another hacky approach like things I did before. So i decided to use his second idea which (after some trial and error) worked perfectly: Using themes themselves.
First off, I had to create a new special theme for one of the entries in the
LayerContainer. Note here that an inherited theme (from a node aboce) could not be found here. Explicitly set the a theme into this entry-node.
In this theme, as stated by tripod "add a custom StyleBox for focus", in my case i called it "FocusedBox". Then "in the _draw() method, check for focus and render the focus box" like this:
var focussed = theme.get_stylebox("FocusedBox", "BoxContainer")
draw_style_box(focussed, Rect2(Vector2(0,0), rect_size))