If the mouse is over a TabItem and this TabItem is currently selected, then the color is not GrayhoverBrush, but HighlightBrush
ControlsHelper.HeaderFontSize is set to Inherited=true in order to allow to set the HeaderFontSize at TabControl level

<TabControl ControlsHelper.HeaderFontSize="20">
....
Revert "special property" behavior
Revert space missmatch
Merge pull request #1547 from xxMUROxx/hotfix/TabItem

[Fix] TabControl, MouseOver & IsSelected Background color