#tabs-newtab-button {
  /* Original:
    margin: 0 0 var(--tabs-navbar-shadow-size) !important
    => Can't ovrride style. Therefore, we should approach it by bypass.
    */
  --tabs-navbar-shadow-size: -1px; /* Original: 1px */
  --tabs-navbar-original-shadow-size: 1px;
  --tab-corner-rounding: 4px; /* Hardcorded */

  /* Size */
  -moz-box-align: stretch !important;
  padding-top: var(--tab-block-margin) !important;

  /* Corner Rounding Image */
  --newtab-position: calc((var(--tab-corner-rounding) / 2) * -1);
  background-image: url("../icons/tab-bottom-corner-left.svg"), url("../icons/tab-bottom-corner-right.svg");
  background-repeat: no-repeat;
  background-position: left var(--newtab-position) bottom var(--tabs-navbar-original-shadow-size),
    right var(--newtab-position) bottom var(--tabs-navbar-original-shadow-size);
  background-size: var(--tab-corner-rounding);

  /* Corner Rounding Color */
  fill: transparent !important;
  -moz-context-properties: fill !important;
}

/* Corner Rounding Color */
#tabs-newtab-button:hover {
  fill: var(--toolbarbutton-hover-background) !important;
}
#tabs-newtab-button:hover:active {
  fill: var(--toolbarbutton-active-background) !important;
}
@media (-moz-windows-accent-color-in-titlebar) {
  :root[tabsintitlebar]:not(:-moz-window-inactive, :-moz-lwtheme) #tabs-newtab-button:hover,
  :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover {
    fill: color-mix(
      in srgb,
      -moz-accent-color-foreground 10%,
      transparent
    ) !important; /* Hardcorded for compatibility */
  }
  :root[tabsintitlebar]:not(:-moz-window-inactive, :-moz-lwtheme) #tabs-newtab-button:hover:active,
  :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover:active {
    fill: color-mix(
      in srgb,
      -moz-accent-color-foreground 15%,
      transparent
    ) !important; /* Hardcorded for compatibility */
  }
}

/* '+'Icon */
#tabs-newtab-button .toolbarbutton-icon {
  border-radius: var(--tab-border-radius) var(--tab-border-radius) 0 0 !important; /* Original: var(--tab-border-radius) */

  padding: calc(var(--toolbarbutton-inner-padding) - (var(--tab-block-margin) / 4)) var(--toolbarbutton-inner-padding)
    calc(var(--toolbarbutton-inner-padding) + (var(--tab-block-margin) / 4) + var(--tabs-navbar-original-shadow-size)) !important;
  -moz-context-properties: fill, fill-opacity;
  fill: var(--toolbarbutton-icon-fill);
  fill-opacity: var(--toolbarbutton-icon-fill-opacity);
}