gnome-shell/data/theme/gnome-shell-sass/widgets/_panel.scss

/* Top Bar */
// a.k.a. the panel

$panel_corner_radius: $base_border_radius+1;
$panel_bg_color: #000;
$panel_fg_color: #ddd;
$panel_height: 2.2em;
$panel_transition_duration: 250ms; // same as the overview transition duration

#panel {
  background-color: $panel_bg_color;
  font-weight: bold;
  height: $panel_height;
  font-feature-settings: "tnum";
  transition-duration: $panel_transition_duration;

  // transparent panel on lock & login screens
  &.unlock-screen,
  &.login-screen,
  &:overview {
    background-color: transparent;

    .panel-corner {
      -panel-corner-opacity: 0;
    }
  }

  // spacing between activities, app menu and such
  #panelLeft, #panelCenter {
    spacing: $base_spacing;
  }

  // the rounded outset corners
  .panel-corner {
    -panel-corner-radius: $panel_corner_radius;
    -panel-corner-background-color: $panel_bg_color;
    -panel-corner-border-width: 2px;
    -panel-corner-border-color: transparent;
    -panel-corner-opacity: 1;
    transition-duration: $panel_transition_duration;
  }

  // panel menus
  .panel-button {
    font-weight: bold;
    color: $panel_fg_color;
    -natural-hpadding: $base_padding * 2;
    -minimum-hpadding: $base_padding;
    transition-duration: 150ms;
    border: 3px solid transparent;
    border-radius: 99px;

    &.clock-display {
      .clock {
        transition-duration: 150ms;
        border: 3px solid transparent;
        border-radius: 99px;
      }
    }

    &:hover, &:active, &:overview, &:focus, &:checked {
      box-shadow: inset 0 0 0 100px rgba(255, 255, 255, 0.20);

      // The clock display needs to have the background on .clock because
      // we want to exclude the do-not-disturb indicator from the background
      &.clock-display {
        box-shadow: none;

        .clock {
          box-shadow: inset 0 0 0 100px rgba(255, 255, 255, 0.20);
        }
      }
    }

    // status area icons
    .system-status-icon {
      icon-size: $base_icon_size;
      padding: $base_padding - 1px;
      margin: 0 $base_margin;
    }

    .panel-status-indicators-box .system-status-icon,
    .panel-status-menu-box .system-status-icon {
      margin: 0;
    }

    // app menu icon
    .app-menu-icon {
      -st-icon-style: symbolic;
      // dimensions of the icon are hardcoded
    }

    &#panelActivities {
      -natural-hpadding: $base_padding * 3;
    }
  }

  &.unlock-screen,
  &.login-screen,
  &:overview {
    .panel-button {
      &:hover, &:active, &:overview, &:focus, &:checked {
        box-shadow: inset 0 0 0 100px rgba(255, 255, 255, 0.15);

        &.clock-display {
          box-shadow: none;

          .clock {
            box-shadow: inset 0 0 0 100px rgba(255, 255, 255, 0.15);
          }
        }
      }
    }
  }

  .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;
  }

  // indicator for active
  .screencast-indicator,
  .remote-access-indicator { color: $warning_color; }
}

// App Menu
#appMenu {
  spacing: $base_spacing;
  .label-shadow { color: transparent; }
}

#appMenu .panel-status-menu-box {
  padding: 0 $base_padding;
  spacing: $base_spacing;
}