Fix missing tab background in new TST version

Author: not-holar

tree_style_tabs/extra_style_rules.css

@@ -3,6 +3,10 @@ tab-item .closebox {
 	visibility: collapse;
 }
 
+tab-item {
+	--tab-surface: hsla(0, 0%, 100%, .05);
+}
+
 tab-item {
 	border: none;
 	outline: 1px solid hsla(0, 0%, 0%, 0.09);