Document DBase4_3DTheme in THEMES.md

[sfloess]

Summary

DBase4_3DTheme was added in commit d213790 but is not documented in THEMES.md.

Missing Documentation

• Historical context (dBASE IV Control Center)
• Color scheme details (white on blue, yellow menus)
• 3D features explanation (shadows, raised/sunken)
• Usage examples
• Visual comparison with Borland3DTheme
• When to use this theme

Location

Should be added after Borland3DTheme section in THEMES.md

Acceptance Criteria

• Complete section in THEMES.md
• Usage code examples
• Historical context
• Color scheme table
• Visual comparison with Borland3DTheme