Update default color scheme and layout to modern monochrome look

[jjroelofs]

Proposal

The current DXPR Theme default look and feel uses a blue/green color scheme with boxed layout, fixed/sticky header, and other settings that feel dated. This issue proposes updating the defaults to a modern, clean monochrome aesthetic.

Color changes

• Rename the current "default" scheme to "classic" so existing users can still select it
• Replace the default scheme with a monochrome black-and-white palette (black primary, dark gray secondary, white backgrounds)

Layout and typography changes

• Disable fixed/sticky header (header_top_fixed: 0, header_top_sticky: 0)
• Disable boxed layout (boxed_layout: 0)
• Update scale_factor from 1.34 to 1.25
• Update headings_line_height from 1.1 to 1.2
• Change hamburger_menu from three-dash to two-dash
• Change menu_hover from opacity to text
• Set block_card to "card card-body"

Backward compatibility

• A post-update hook migrates existing sites from "default" to "classic" so their appearance is preserved
• Only new installations get the updated defaults