Add ThemeManager.useDBase4_3DTheme() convenience method

[sfloess]

Summary

DBase4_3DTheme was added in commit d213790 but ThemeManager is missing the convenience method.

Missing

• useDBase4_3DTheme() method in ThemeManager.java
• JavaDoc for the method
• Entry in Available Themes list

Pattern

Should follow the same pattern as useBorland3DTheme()

Acceptance Criteria

• useDBase4_3DTheme() method exists
• JavaDoc explains dBASE IV 3D theme
• Method listed in ThemeManager class-level JavaDoc
• Tests can use ThemeManager.useDBase4_3DTheme()