WhiteSur-gtk-theme/src/sass/gnome-shell/widgets/_panel.scss

#panel {
  background-color: $panel_bg;
  /* transition from solid to transparent */
  transition-duration: 250ms;
  font-weight: bold;
  height: $menuitem_size;
  box-shadow: 0 3px 6px rgba(black, 0.22), 0 5px 16px rgba(black, 0.35);

  &:overview,
  &.unlock-screen,
  &.login-screen,
  &.lock-screen {
    background-color: transparent;

    .panel-corner {
      -panel-corner-radius: 0;
      -panel-corner-background-color: transparent;
      -panel-corner-border-color: transparent;
    }
  }

  #panelLeft, #panelCenter { // spacing between activities<>app menu and such
    spacing: 0;
  }

  .panel-corner {
    -panel-corner-radius: $panel-corner-radius;
    -panel-corner-background-color: $panel_bg;
    -panel-corner-border-width: 2px;
    -panel-corner-border-color: transparent;

    &:active, &:overview, &:focus {
      -panel-corner-border-color: $panel_fg;
    }

    &.lock-screen, &.login-screen, &.unlock-screen {
      -panel-corner-radius: 0;
      -panel-corner-background-color: transparent;
      -panel-corner-border-color: transparent;
    }

    StLabel { padding: 0 4px; }
  }

  .panel-button {
    -natural-hpadding: 8px;
    -minimum-hpadding: 8px;
    font-weight: bold;
    color: $panel_fg;
    transition-duration: $longer_duration;
    border-radius: $bt_radius;
    // margin-top: 2px;
    // margin-bottom: 2px;

    StLabel { padding: 0 2px; }

    &, &:hover, &:active, &:overview, &:focus, &:checked {
      text-shadow: $panel_asset_shadow;

      .system-status-icon,
      .app-menu-icon > StIcon,
      .popup-menu-arrow {
        icon-shadow: $panel_asset_shadow;
      }
    }

    .app-menu-icon {
      -st-icon-style: symbolic;
      margin-left: 0;
      margin-right: 0;
      icon-size: 16px;
      //dimensions of the icon are hardcoded

      > StIcon {
        padding-left: 4px;
        padding-right: 6px;
      }
    }

    .popup-menu-arrow { width: 0; height: 0; } // Remove arrow on panel button

    &:hover {
      color: $panel_fg;
      background-color: $light_divider_color;
    }

    &:active, &:overview, &:focus, &:checked {
      &, &:hover {
        background-color: $light_track_color;
        color: $panel_fg;
        box-shadow: none;
      }
    }

    .system-status-icon { icon-size: em(16px); padding: 0 4px; }
    .unlock-screen &,
    .login-screen &,
    .lock-screen & {
      color: $panel_fg;
      &:focus, &:hover, &:active { color: $panel_fg; }
    }
  }

  .panel-button.clock-display {
    &:active, &:overview, &:focus, &:checked {
      .clock {
        background: none;
        box-shadow: none;
      }
    }
  }

  .panel-status-indicators-box,
  .panel-status-menu-box {
    spacing: 2px;
  }

  // spacing between power icon and (optional) percentage label
  .power-status.panel-status-indicators-box {
    spacing: 0;
  }

  .screencast-indicator { color: $error_color; }
  .remote-access-indicator { color: $warning_color; } // > 3.29.4

  &.solid {
    background-color: $panel_bg;
    /* transition from transparent to solid */
    transition-duration: 250ms;
    background-gradient-direction: none; // for Ubuntu session
    text-shadow: none; // for Ubuntu session

    &:overview { background-color: transparent; } // for Ubuntu session

    .panel-corner {
      -panel-corner-background-color: $panel_bg;
    }

    .panel-button {
      color: $panel_fg;
      text-shadow: none;

      &:active, &:overview, &:focus, &:checked {
        color: $light_alt_fg_color;
        text-shadow: 0 1px rgba(black, 0.45);
      }
    }

    .system-status-icon,
    .app-menu-icon > StIcon,
    .popup-menu-arrow {
      icon-shadow: none;

      &:active, &:overview, &:focus, &:checked {
        icon-shadow: 0 1px rgba(black, 0.45);
      }
    }
  }
}

// Activities button
#panel #panelActivities.panel-button {
  > * {
    background-image: url("assets/activities.svg");
    background-position: center top;
    // background-size: 18px 18px;
    width: 24px;
    height: 24px;
    background-color: transparent !important;
    background-gradient-direction: none !important;
    border: none;
    color: transparent;
  }

  &:active, &:overview, &:focus, &:checked {
    background-color: transparent;
    border: none;

    // > * { background-image: url("assets/activities-active.svg"); }
  }
}