* Update dark theme tree color and tab bar background
Changed the TreeActiveColor value in the FreeCAD Dark preference pack for improved visibility. Set the QTabBar background color to use @PrimaryColor in the stylesheet for fixing drawbase issues.
* Differentiate QTabBar styles for mdiAreaTabBar
Separated the QTabBar#mdiAreaTabBar styling from the general QTabBar, setting qproperty-drawBase and background-color specifically for mdiAreaTabBar. The general QTabBar now has qproperty-drawBase set to 0, improving style specificity for file tab bars.
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