Theming improvements:

- reworked core themes
- reduced explicit colors by using color functions
- made it easier to create new themes
- avoid using colors from one component type for another component type
  (except when useful; e.g. `Separator.foreground` is also used for other separators)
- HelpButton: use colors from Button (instead of from CheckBox.icon)
- ToolBar: improved "docking" and "floating" colors

flatlaf-testing/src/main/resources/com/formdev/flatlaf/testing/FlatTestLaf.properties

@@ -32,6 +32,8 @@
 @cellFocusColor = #f00
 @icon = #afafaf
 
+@accentUnderlineColor = #f00
+
 # for buttons within components (e.g. combobox or spinner)
 @buttonArrowColor = #666
 @buttonDisabledArrowColor = #ABABAB
@@ -170,6 +172,7 @@ FormattedTextField.focusedBackground = #ff8
 
 HelpButton.focusedBackground = #0ff
 HelpButton.questionMarkColor = #00f
+HelpButton.disabledQuestionMarkColor = #88f
 
 
 #---- InternalFrame ----
@@ -216,7 +219,7 @@ MenuBar.borderColor = #44f
 MenuBar.hoverBackground = #fdd
 
 MenuBar.underlineSelectionBackground = #0f0
-MenuBar.underlineSelectionColor = #f00
+MenuBar.underlineSelectionColor = #ff0
 MenuBar.underlineSelectionHeight = 5
 
 #---- MenuItemCheckBox ----
@@ -301,6 +304,7 @@ Slider.disabledThumbColor = #880
 
 #---- Spinner ----
 
+Spinner.buttonBackground = #cccccc
 Spinner.focusedBackground = #ff8