UI: Theme: Add Curve widget theme settings

Add theme settings for the Curve and CurveProfile widgets.

Even though the curve widget is used all over Blender, it never had its
own theme settings. It was using a mix of colors from the “regular”
toggle widget with text colors inherited from the editor.

Thanks to the recent cleanup and removal of so many redundant theme
options, we can now add new/missing settings without overwhelming
the theme.

See PR for details and screenshots.

Pull Request: https://projects.blender.org/blender/blender/pulls/146274

scripts/startup/bl_ui/space_userpref.py

@@ -1455,6 +1455,7 @@ class ThemeGenericClassGenerator:
     def generate_panel_classes_for_wcols():
         wcols = [
             ("Box", "wcol_box"),
+            ("Curve", "wcol_curve"),
             ("List Item", "wcol_list_item"),
             ("Menu", "wcol_menu"),
             ("Menu Background", "wcol_menu_back"),