/* Curve the background tabs at the upper left and right corners
   May not work on add-on themes. */
.tabbrowser-tab {
  border-radius: 10px 10px 0px 0px / 15px 15px 0px 0px !important;
}
/* Curve active tab and slightly overlap neighboring tabs */
.tabbrowser-tab .tab-background[selected="true"] {
  margin-left: -1px !important;
  margin-right: -1px !important;
  border-radius: 10px 10px 0px 0px / 15px 15px 0px 0px !important;
}
/* Shorten the tab line along the tops of tabs */
.tab-line {
  margin-left: 9px !important;
  margin-right: 9px !important;
}
/* Shorten the vertical lines between background tabs */
.tabbrowser-tab::before, .tabbrowser-tab::after {
  margin-top: 9px !important;
}

/** "Light" Theme (Dark Text) **/

/* Border the background tabs (10% gray) for greater visibility */
#main-window:not([lwthemetextcolor="bright"]) .tabbrowser-tab:not([selected="true"]) {
  border: 1px solid rgba(0,0,0,.1) !important;
  border-bottom-color: transparent !important;
  margin-right: -1px !important;
}
/* Remove short vertical lines between background tabs */
#main-window:not([lwthemetextcolor="bright"]) .tabbrowser-tab::before, 
#main-window:not([lwthemetextcolor="bright"]) .tabbrowser-tab::after {
  border: none !important;
}