Add Steam Machine theme to extra themes

[ozdil]

Summary

Adds the Steam Machine (Special Edition) theme to the Extra Themes page.

• Repository: https://github.com/ozdil/omarchy-steammachine-theme
• Theme Details: Special edition, eye-friendly, OLED-optimized dark theme celebrating Valve, Steam Deck, Half-Life, and Aperture Science. Steam gunmetal black background with electric cyan blue, Valve industrial red, and Half-Life lambda orange accents.
• Includes: 8K OLED vector wallpapers featuring Steam Machine mechanical piston logo, Steam Deck & Controller, and Valve universe tribute.