If you must have the classic library in dark mode, you can use Windows settings, though this affects all apps on your computer. Go to Settings > Accessibility > Contrast Themes . Select a dark theme like Aquatic or Dusk . Windows Media Player Legacy - Microsoft Support
Tech Support: How to change the Windows Media Player 12 Skin 7 Jul 2012 — windows media player 12 dark theme
Change the value (default is 6) to another number to see different color options. If you must have the classic library in
Suddenly, the screen flickered. The familiar bright blue and white bars dissolved. In their place emerged a deep, obsidian finish with neon-blue accents that pulsed with the beat of his lo-fi playlist. The buttons felt tactile, the library glowed like a futuristic terminal, and for the first time, his music felt at home in the dark. Windows Media Player Legacy - Microsoft Support Tech
: Select the skin in the left pane and click Apply Skin . 2. Modify the Library Background (Registry Tweak)