hugo-theme-hello-friend/source/css/header.css

.header {
  background: #fafafa;
  display: flex;
  align-items: center;
  justify-content: center;
  position: relative;
  padding: 20px;

  .dark-theme & {
    background: #252627;
  }

  .bulb {
    &-on {
      display: none;

      .dark-theme & {
        display: block;
      }
    }

    &-off {
      display: block;

      .dark-theme & {
        display: none;
      }
    }
  }

  &__right {
    display: flex;
    flex-direction: row;
    align-items: center;

    @media (--phone) {
      flex-direction: row-reverse;
    }
  }

  &__inner {
    display: flex;
    align-items: center;
    justify-content: space-between;
    margin: 0 auto;
    width: 760px;
    max-width: 100%;
  }
}

.theme-toggle {
  cursor: pointer;
}