Restore GTK-informed widget styling for menupopups on linux

Firefox restyled Linux menupopups at some point to no longer conform to
the DE defined styles for GTK widgets. This removes that custom firefox
styling and uses DE defined values instead. This is especially apparent
on Linux Unity where menupopups are dark in both light and dark themes
by default for apps.

scss/linux/_menu.scss

@@ -0,0 +1,13 @@
+@each $cls, $icon in $menu-icons {
+	.zotero-menuitem-#{$cls} {
+		// If icon starts with "light-dark:", use light and dark icon
+		@if str-slice($icon, 0, 11) == "light-dark:" {
+			@include focus-states using ($color) {
+				@include svgicon-menu(str-slice($icon, 12), $color, "16");
+			}
+		}
+		@else {
+			@include svgicon-menu($icon, "universal", "16", false, false, false);
+		}
+	}
+};

scss/linux/_menupopup.scss

@@ -1,10 +1,7 @@
 menupopup, panel, #search-mode-popup {
-	--panel-padding-block: 0;
-	--panel-border-radius: 6px;
-	--panel-background: var(--material-toolbar);
-	--menuitem-hover-background-color: var(--fill-quinary);
-	--menuitem-disabled-hover-background-color: var(--fill-quinary);
-	@include light-dark(--panel-border-color, #dddddd, #1c1c1c);
+	--panel-padding-block: initial;
+	--panel-border-color: initial;
+	--panel-border-radius: 3px;
 }
 
 menupopup {
@@ -16,5 +13,5 @@ menupopup {
 }
 
 :is(panel, menupopup)::part(content) {
-	padding: 6px 0px 6px 0px;
+	padding: 3px 0;
 }

scss/linux/_titleBar.scss

@@ -15,6 +15,10 @@
 	-moz-appearance: -moz-window-titlebar;
 }
 
+#zotero-title-bar {
+	border-top: 1px solid var(--toolbar-field-border-color);
+}
+
 #toolbar-menubar {
 	pointer-events: none;
 	visibility: visible;

scss/zotero-unix.scss

@@ -8,6 +8,7 @@
 @import "linux/editable";
 @import "linux/errorReport";
 @import "linux/feedSettings";
+@import "linux/menu";
 @import "linux/menupopup";
 @import "linux/tabBar";
 @import "linux/titleBar";