session-desktop/stylesheets/_session_theme_dark.scss

// Messages
.dark-theme {
  .module-conversation-header {
    border-bottom: none;
    background-color: $session-shade-4;
  }
  .discussion-container {
    background-image: $session-gradient-black;
  }

  .module-message {
    &__author,
    &__metadata__badge,
    &__metadata__date--incoming,
    &__metadata__date--outgoing {
      @include session-color-subtle($session-color-white);
    }

    &__container--incoming {
      background-color: $session-shade-11;
    }

    &__container--outgoing {
      background-color: $session-shade-5;
    }

    &__container {
      transition: background-color 0.25s;
      box-shadow: 0px 0px 8px 2px rgba(0, 0, 0, 0.69);
    }
  }

  .message {
    &-highlighted {
      background-color: $session-shade-5;
      border-radius: 0;
    }

    &-selected {
      .module-message {
        &__container {
          background-image: $session-gradient-green;
          box-shadow: $session-dark-shadow;
        }

        &__author {
          color: $session-color-white;
        }
      }
    }

    &-read-receipt-container {
      margin-left: 5px;
    }
  }
}