I have a horizontal centrally aligned (using a spacer) toolbar at the bottom of dual listers like so -
When I maximize the window the toolbar does not stay centralized, is there a way to do this please. If not, add it to the feature requests list 
Does it work if you have a spacer on both the left and right end of the buttons of that toolbar? Can you show us what it looks like maximized?
If you have a full width spacer on both sides of the toolbar, the stuff between them will always be centred.
My guess is there's a fixed width spacer on either or both sides at the moment.
You can set the "full width" option on spacers via their right-click menu while in Customize mode.