Windows Media Player 12 (the classic version) does not have a native, built-in "Dark Theme" toggle . While modern Windows apps follow system-wide dark mode settings, this legacy player remains largely locked to its traditional light blue/white aesthetic.
Use an alternative media player with native dark mode windows media player 12 dark theme
If you don’t want to change your entire Windows theme, third-party skins are your best bet. WMP 12 supports skin files ( .wmz or .wms ). Over the years, the community has created stunning dark themes. Windows Media Player 12 (the classic version) does