session-desktop/stylesheets/_themes.scss

@mixin invert-text-color {
  &, .meta {
    color: white;
  }

  .content {
    &::selection, a::selection {
      background: white;
      color: $grey_d;
    }

    &::-moz-selection, a::-moz-selection {
      background: white;
      color: $grey_d;
    }
  }

  .attachments, .content {
    a {
      color: $grey_l;
    }
  }
}

.ios {
  .incoming .bubble {
    background-color: $grey_l;
    color: $grey_d;
  }
  .outgoing .bubble {
    background-color: $blue;
    @include invert-text-color;
  }
}

.android {
  .outgoing .bubble {
    background-color: $grey_l;
  }

  .incoming .bubble {
    @include invert-text-color;
  }
}

.avatar, .conversation-header, .android .bubble {
  &.red         { background-color: $material_red         ; }
  &.pink        { background-color: $material_pink        ; }
  &.purple      { background-color: $material_purple      ; }
  &.deep_purple { background-color: $material_deep_purple ; }
  &.indigo      { background-color: $material_indigo      ; }
  &.blue        { background-color: $material_blue        ; }
  &.light_blue  { background-color: $material_light_blue  ; }
  &.cyan        { background-color: $material_cyan        ; }
  &.teal        { background-color: $material_teal        ; }
  &.green       { background-color: $material_green       ; }
  &.light_green { background-color: $material_light_green ; }
  &.orange      { background-color: $material_orange      ; }
  &.deep_orange { background-color: $material_deep_orange ; }
  &.amber       { background-color: $material_amber       ; }
  &.blue_grey   { background-color: $material_blue_grey   ; }
  &.grey        { background-color: #999999               ; }
  &.default     { background-color: $blue                 ; }
}