Add dark/light theme support with improved icon

This implements dark/light theme switching for guiconfig with a blue
color scheme, replacing the misleading green X window icon with a gear
icon that better represents the configuration tool.
- Force clam theme for consistent cross-platform styling
- Pure ttk architecture (removed hybrid tk/ttk complexity)
- Keyboard shortcut: Ctrl+T to toggle themes
- Shape: Green X → Blue 8-tooth gear
- Reorganized toolbar: Theme button integrated into View Options
- Responsive layout: Grid column weights for proper resizing
- Text widget theming: Dark mode support for description area

Author: jserv