Thunderbird_91_ESR/css/tabs/classic_squared_tabs.css
/* Thunderbird userChrome.css tweaks ************************************************************/
/* Github: https://github.com/aris-t2/customcssfortb ********************************************/


/* remove top toolbar shadows */
#mail-toolbox,
.mail-toolbox,
#mail-toolbar-menubar2,
#tabs-toolbar,
#tabs-toolbar::after,
#tabs-toolbar::before,
#navigation-toolbox::before,
#navigation-toolbox::after {
  box-shadow: none !important;
}

/* remove curved tabs look */
.tabmail-tab .tab-background,
.tab-background-start[selected=true]:-moz-locale-dir(ltr)::before,
.tab-background-end[selected=true]:-moz-locale-dir(rtl)::before,
.tab-background-end[selected=true]:-moz-locale-dir(ltr)::before,
.tab-background-start[selected=true]:-moz-locale-dir(rtl)::before,
.tab-background-start[selected=true]:-moz-locale-dir(ltr)::after,
.tab-background-end[selected=true]:-moz-locale-dir(rtl)::after,
.tab-background-end[selected=true]:-moz-locale-dir(ltr)::after,
.tab-background-start[selected=true]:-moz-locale-dir(rtl)::after,
.tab-background-middle[selected=true]{
  background: none !important;
  box-shadow: unset !important;
}

/* remove useless tab text/label indicator */
.tabmail-tab[selected] .tab-text,
.tabmail-tab[selected] .tab-label {
  border: 1px solid transparent !important;
}

/* adjust tab image */
.tab-image-left,
.tab-image-right {
  width: 8px !important;
  clip-path: none !important;
  background:none !important;
}

/* tab icon position */
.tab-throbber,
.tab-icon-image {
  margin-inline-start: -6px !important;
}
.tab-close-button {
  margin-inline-end: -6px !important;
}

/* remove tab separators */
.tabmail-tab:not([selected]):not(:hover) +
.tabmail-tab:not([selected]):not(:hover)::before,
.tabmail-tab:last-child:not([selected]):not(:hover)::after {
  /*visibility:collapse !important;*/
  opacity: 0 !important;
}

/* remove forced tab height */
.tab-background-start[selected=true]::after,
.tab-background-start[selected=true]::before,
.tab-background-start,
.tab-background-end,
.tab-background-end[selected=true]::after,
.tab-background-end[selected=true]::before {
  min-height: 0px !important;
}

/* remove tab background margin */
.tab-background {
  margin-inline-end: 0px !important;
  margin-inline-start: 0px !important;
}

.tab-background-middle {
  margin-inline-end: 0px !important;
  margin-inline-start: 0px !important;
}

.tab-background-start,
.tab-background-end {
  width: 0 !important;
  margin-inline-end: 0px !important;
  margin-inline-start: 0px !important;
}

/* remove tab scrollbox padding */
.tabmail-arrowscrollbox > scrollbox {
  padding-inline-end: unset !important;
  padding-inline-start: unset !important;
}

/* tab size */
.tabmail-tab {
  appearance: none !important;
  margin: 0px 0px 1px 0px !important;
  min-height: 0px !important;
}

/* remove line for light lw-themes */
#tabs-toolbar:-moz-lwtheme-darktext {
  background: unset !important;
}

/* override tab text colors */
#tabs-toolbar:not(:-moz-lwtheme) .tabmail-tab,
#tabs-toolbar:not(:-moz-lwtheme) .tabmail-tab > .tab-stack > .tab-content > .tab-text,
#tabs-toolbar:-moz-lwtheme-darktext .tabmail-tab,
#tabs-toolbar:-moz-lwtheme-darktext .tabmail-tab > .tab-stack > .tab-content > .tab-text {
  color: black !important;
}

#tabs-toolbar:-moz-lwtheme-brighttext .tabmail-tab,
#tabs-toolbar:-moz-lwtheme-brighttext .tabmail-tab > .tab-stack > .tab-content > .tab-text {
  color: white !important;
}

/* non-Aero: e.g Win Classic, Linux or macOS*/
/*@media not all and (-moz-windows-theme:aero) {*/
	
	/* tab appearance */
	.tabmail-tab {
	  border: 1px solid #808080 !important;
	  border-bottom: 0px !important;
	}

	#tabs-toolbar:-moz-lwtheme-brighttext .tabmail-tab {
	  border: 1px solid black !important;
	  border-bottom: 0px !important;
	}

	#tabs-toolbar:not(:-moz-lwtheme) .tabmail-tab {
	  background-image: linear-gradient(transparent,hsla(0,0%,45%,.1) 1px, hsla(0,0%,32%,.2) 80%, hsla(0,0%,0%,.2)),
						linear-gradient(to top,rgba(0,0,0,.4),-moz-dialog 1px, -moz-dialog) !important;
	}

	#tabs-toolbar:not(:-moz-lwtheme) .tabmail-tab:hover{
	  background-image: linear-gradient(transparent,hsla(0,0%,100%,.3) 1px, hsla(0,0%,75%,.2) 80%, hsla(0,0%,60%,.2)),
						linear-gradient(to top,rgba(0,0,0,.4),-moz-dialog 1px, -moz-dialog) !important;
	}

	#tabs-toolbar:not(:-moz-lwtheme) .tabmail-tab[selected] {
	  background-image: linear-gradient(rgba(255,255,255,.7), rgba(255,255,255,.4) 50%),
						linear-gradient(-moz-dialog, -moz-dialog) !important;
	}

	#tabs-toolbar:-moz-lwtheme .tabmail-tab {
	  background-image: linear-gradient(hsla(0,0%,100%,.3) 1px, hsla(0,0%,75%,.2) 80%, hsla(0,0%,60%,.2)) !important;
	}

	#tabs-toolbar:-moz-lwtheme .tabmail-tab:hover{
	  background-image: linear-gradient(rgba(255,255,255,.5), rgba(255,255,255,.3) 50%) !important;
	}

	#tabs-toolbar:-moz-lwtheme .tabmail-tab[selected] {
	  background-image: linear-gradient(rgba(255,255,255,.9), rgba(255,255,255,.4) 50%) !important;
	}
	
	/* tab close icon appearance */
	.tabmail-tab:not([selected]) .tab-close-button {
	  list-style-image: url('../../images/close.png') !important;
	 -moz-image-region: rect(0, 16px, 16px, 0) !important;
	}

	.tabmail-tab:not([selected]) .tab-close-button:hover,
	.tabmail-tab:not([selected]) .tab-close-button:hover[selected="true"] {
	  -moz-image-region: rect(0, 32px, 16px, 16px) !important;
	}

	.tabmail-tab:not([selected]) .tab-close-button:hover:active,
	.tabmail-tab:not([selected]) .tab-close-button:hover:active[selected="true"] {
	  -moz-image-region: rect(0, 48px, 16px, 32px) !important;
	}
/*}*/


/* Windows 'Aero' */
@media all and (-moz-windows-theme:aero) {

	/* tab appearance */
	.tabmail-tab {
	  border: 1px solid rgba(0,0,0,.4) !important;
	  border-bottom: 0px !important;
	}

	#tabs-toolbar:-moz-lwtheme-brighttext .tabmail-tab {
	  border: 1px solid black !important;
	  border-bottom: 0px !important;
	}
	
	/* Win8/Win10 */
	@media (-moz-os-version:windows-win8),(-moz-os-version:windows-win10) {
		
		/* tab appearance */
		#tabs-toolbar .tabmail-tab {
		  border: 1px solid rgba(0,0,0,.2) !important;
		  border-bottom: 0px !important;
		}

		#tabs-toolbar:-moz-lwtheme-brighttext .tabmail-tab {
		  border: 1px solid black !important;
		  border-bottom: 0px !important;
		}

		#tabs-toolbar:not(:-moz-lwtheme) .tabmail-tab {
		  background-image: linear-gradient(transparent, hsla(0,0%,45%,.1) 1px, hsla(0,0%,32%,.2) 80%, hsla(0,0%,0%,.2)),
							linear-gradient(to top,rgba(0,0,0,.4),-moz-dialog 1px, -moz-dialog) !important;
		}

		#tabs-toolbar:not(:-moz-lwtheme) .tabmail-tab:hover{
		  background-image: linear-gradient(transparent, hsla(0,0%,100%,.3) 1px, hsla(0,0%,75%,.2) 80%, hsla(0,0%,60%,.2)),
							linear-gradient(to top,rgba(0,0,0,.4),-moz-dialog 1px, -moz-dialog) !important;
		}

		/*#tabs-toolbar:not(:-moz-lwtheme) .tabmail-tab[selected] {
		  background-image: linear-gradient(rgba(255,255,255,.7), rgba(240,240,240,.4) 50%),
							linear-gradient(-moz-dialog, -moz-dialog) !important;
		}*/
		
		#tabs-toolbar:not(:-moz-lwtheme) {
		  background-image: linear-gradient(to top, rgba(0,0,0,.6), transparent 1px) !important;
		}
		
		/* tab close icon appearance */
		.tabmail-tab .tab-content  .tab-close-button .toolbarbutton-icon,
		.tabmail-tab .tab-content .close-icon .toolbarbutton-icon{
		  /*min-width: unset !important;
		  width: 16px !important;*/
		}
		
	}
	
	@media (-moz-os-version:windows-win8) {
		
		#tabs-toolbar:not(:-moz-lwtheme) .tabmail-tab[selected] {
		  background-image: linear-gradient(rgba(255,255,255,.7), rgba(240,240,240,.0) 50%),
							linear-gradient(-moz-dialog, #f9f9fa) !important;
		}
	
	}
	
	@media (-moz-os-version:windows-win10) {
		
		#tabs-toolbar:not(:-moz-lwtheme) .tabmail-tab[selected] {
		  background-image: linear-gradient(rgba(255,255,255,.7), rgba(240,240,240,.4) 50%),
							linear-gradient(-moz-dialog, -moz-dialog) !important;
		}
	
	}

}


/* default Dark lw-theme */
#messengerWindow:not([style*='--lwt-header-image']):-moz-lwtheme:-moz-lwtheme-brighttext #tabs-toolbar .tabmail-tab {
  background-image: linear-gradient(#0c0c0d, #0c0c0d) !important;
}

#messengerWindow:not([style*='--lwt-header-image']):-moz-lwtheme:-moz-lwtheme-brighttext #tabs-toolbar .tabmail-tab:hover{
  background-image: linear-gradient(#252526, #252526) !important;
}

#messengerWindow:not([style*='--lwt-header-image']):-moz-lwtheme:-moz-lwtheme-brighttext #tabs-toolbar .tabmail-tab[selected] {
  background-image: linear-gradient(#323234, #323234) !important;
}

/* default Light lw-theme */
#messengerWindow:not([style*='--lwt-header-image']):-moz-lwtheme:-moz-lwtheme-darktext #tabs-toolbar .tabmail-tab {
  background-image: linear-gradient(#e3e4e6, #e3e4e6) !important;
}

#messengerWindow:not([style*='--lwt-header-image']):-moz-lwtheme:-moz-lwtheme-darktext #tabs-toolbar .tabmail-tab:hover{
  background-image: linear-gradient(#cccdcf, #cccdcf) !important;
}

#messengerWindow:not([style*='--lwt-header-image']):-moz-lwtheme:-moz-lwtheme-darktext #tabs-toolbar .tabmail-tab[selected] {
  background-image: linear-gradient(#f5f6f7, #f5f6f7) !important;
}

@media (min-resolution: 1.0dppx) {
	
  /* tab close icon appearance */
  #messengerWindow:not(:-moz-lwtheme) .tabmail-tab .close-icon {
	list-style-image: url("../../images/close.png") !important;
	-moz-image-region: rect(0, 16px, 16px, 0) !important;
  }

  #messengerWindow:not(:-moz-lwtheme) .tabmail-tab .close-icon:hover {
	-moz-image-region: rect(0, 32px, 16px, 16px) !important;
  }

  #messengerWindow:not(:-moz-lwtheme) .tabmail-tab .close-icon:hover:active {
	-moz-image-region: rect(0, 48px, 16px, 32px) !important;
  }

  #messengerWindow:not(:-moz-lwtheme) .tabmail-tab .close-icon > .button-icon,
  #messengerWindow:not(:-moz-lwtheme) .tabmail-tab .close-icon > .button-box > .button-icon,
  #messengerWindow:not(:-moz-lwtheme) .tabmail-tab .close-icon > .toolbarbutton-icon {
	/*width: 16px !important;*/
  }

}


/* Tb 57 tweaks */

.tabmail-tab {
  margin: 0px 0px 0px 0px !important;
}

.tabmail-tab[selected] .tab-text,
.tabmail-tab[selected] .tab-label {
  border: 0px solid transparent !important;
}

/* tab icon position */
.tab-throbber,
.tab-icon-image {
  margin-inline-start: -6px !important;
}
.tab-close-button {
  margin-inline-end: -8px !important;
}

/* remove fckng rtrdd space before tabs, seriously devs what is wrong with you? */
:root[tabsintitlebar][sizemode=normal] #mail-toolbar-menubar2[autohide=true] ~ #tabs-toolbar > #tabbar-toolbar {
  margin-inline-start: 0px !important;
}

:root[tabsintitlebar][sizemode=normal] #mail-toolbar-menubar2[autohide=true] ~ #tabs-toolbar {
  margin-top: 2px !important;
}

/* remove tab toolbars box shadow */
#tabs-toolbar:not(:-moz-lwtheme)::before {
  box-shadow: unset !important;
}

/* unset default tab colors */
#tabs-toolbar .tabmail-tab[visuallyselected="true"],
#tabs-toolbar .tabmail-tab,
#tabs-toolbar .tabmail-tab .tab-background {
  background: unset !important;
}

/* unset default tab colors */
#tabs-toolbar .tabmail-tab .tab-background {
  border: unset !important;
}

/* hide tab separators */
.tabmail-tab::after,
.tabmail-tab::before {
  opacity: 0 !important;
  border-image: unset !important;
  border-image-slice: unset !important;
  width: unset !important;
}


#tabs-toolbar .tabmail-tab .tab-line {
  display: none !important;
  height: 0 !important;
  opacity: 0 !important;
}

.tabmail-tab .close-icon,
.tabmail-tab .close-icon > .button-icon,
.tabmail-tab .close-icon > .button-box > .button-icon,
.tabmail-tab .close-icon > .toolbarbutton-icon {
  /*width: 16px !important;
  height: 16px !important;*/
  background: unset !important;
}

#messengerWindow[sizemode="maximized"] #tabbar-toolbar {
  min-height: 1px !important;
  height: 1px !important;
}

/* fix tabs toolbar position */
:root[tabsintitlebar][sizemode=normal] #tabs-toolbar {
  padding-inline-start: 3px !important;
}
:root[tabsintitlebar][sizemode=maximized] #tabs-toolbar {
  padding-inline-start: 0px !important;
}

/* fix menubar postion */
#mail-toolbar-menubar2 {
  padding-top: unset !important;
  margin-top: unset !important;
}
