Added tab control background

I18N Commander/App/theme/tabs/background_style_box_flat.tres

@@ -0,0 +1,9 @@
+[gd_resource type="StyleBoxFlat" format=3 uid="uid://bumfybajbkybi"]
+
+[resource]
+bg_color = Color(0.2, 0.2, 0.2, 1)
+corner_radius_top_left = 12
+corner_radius_top_right = 12
+expand_margin_left = 3.0
+expand_margin_top = 12.0
+expand_margin_right = 12.0