List, Table, Tree: improved color of cell focus indicator border

- was black
- now derived from selection color (usually accent color)
  - darker in light themes
  - lighter in dark themes

(issue #654)

flatlaf-core/src/main/resources/com/formdev/flatlaf/FlatDarkLaf.properties

@@ -57,7 +57,7 @@
 @menuAcceleratorSelectionForeground = @selectionForeground
 
 # misc
-@cellFocusColor = #000
+@cellFocusColor = lighten(@selectionBackground,10%)
 @icon = shade(@foreground,7%)
 
 # accent colors (blueish)

flatlaf-core/src/main/resources/com/formdev/flatlaf/FlatLightLaf.properties

@@ -57,7 +57,7 @@
 @menuAcceleratorSelectionForeground = @selectionForeground
 
 # misc
-@cellFocusColor = #000
+@cellFocusColor = darken(@selectionBackground,20%)
 @icon = shade(@background,27%)
 
 # accent colors (blueish)

flatlaf-core/src/main/resources/com/formdev/flatlaf/IntelliJTheme$ThemeLaf.properties

@@ -49,6 +49,9 @@ infoText = lazy(ToolTip.foreground)
 # make sure that accent color (set via FlatLaf.setSystemColorGetter()) is ignored 
 @accentColor = null
 
+# use fixed color because it is used in borders
+@cellFocusColor = #222
+
 
 #---- Button ----