youtube-music/src/plugins/transparent-player/style.css

:root {
  --ytmd-transparent-player-transparency-color: #111;
  --ytmd-transparent-player-transparent-background: rgb(
    from var(--ytmd-transparent-player-transparency-color) r g b /
      var(--ytmd-transparent-player-opacity, 0.5)
  );
  --ytmd-transparent-player-transparent-background-dark: rgb(
    from var(--ytmd-transparent-player-transparency-color) r g b / 0.8
  );
  --ytmd-transparent-player-backdrop-blur: blur(20px);
}

body.transparent-background-color {
  background-color: var(--ytmd-transparent-player-transparent-background) !important;
}

body.transparent-player-backdrop-filter {
  #layout {
    #nav-bar-background,
    #player-bar-background {
      backdrop-filter: var(--ytmd-transparent-player-backdrop-blur) !important;
    }
  }

  #search-page {
    #tabs {
      &.stuck {
        backdrop-filter: var(--ytmd-transparent-player-backdrop-blur) !important;
      }
    }
  }

  ytmusic-menu-popup-renderer {
    backdrop-filter: var(--ytmd-transparent-player-backdrop-blur) !important;
  }

  #ytmd-title-bar-main-panel {
    backdrop-filter: var(--ytmd-transparent-player-backdrop-blur) !important;
  }
}

body.transparent-player {
  ytmusic-app {
    ytmusic-app-layout[player-page-open] {
      #nav-bar-background.ytmusic-app-layout,
      #player-bar-background.ytmusic-app-layout {
        opacity: 0 !important;
      }
    }

    #layout {
      #nav-bar-background,
      #player-bar-background {
        background: var(--ytmd-transparent-player-transparent-background-dark) !important;
      }

      #mini-guide-background {
        background: none !important;
        border: 0 !important;
      }

      #guide {
        #guide-wrapper {
          background: none !important;
          border: 0 !important;
        }
      }

      ytmusic-player-bar {
        background: none !important;
      }

      #player-page {
        background: none !important;
      }

      #search-page {
        #tabs {
          &.stuck {
            background: var(--ytmd-transparent-player-transparent-background) !important;
          }
        }
      }

      #browse-page {
        #background {
          display: none !important;
        }

        .background-gradient {
          background: none !important;
        }
      }
    }
  }

  /* Window Top Panel */
  nav[data-ytmd-main-panel] {
    background-color: transparent !important;
  }

  /* Video Toggle Plugin */
  .av-toggle.ytmusic-av-toggle {
    background-color: var(--ytmd-transparent-player-transparent-background);
  }
}