Bug 1757506 - Restore selected tab background image. r=dao

This is closer to what the standard lwt dark mode does, and seems fair
enough. I don't mind much either way so if you feel strongly about it
lmk.

Differential Revision: https://phabricator.services.mozilla.com/D139931

browser/themes/linux/browser.css

@@ -18,6 +18,7 @@
   --toolbar-non-lwt-bgcolor: -moz-dialog;
   --toolbar-non-lwt-textcolor: -moz-dialogtext;
   --toolbar-non-lwt-bgimage: linear-gradient(rgba(255,255,255,.075), rgba(255,255,255,.075));
+  --tab-selected-non-lwt-bgimage: linear-gradient(rgba(255,255,255,.15), rgba(255,255,255,.15));
 
   --toolbarbutton-border-radius: 4px;
   --toolbarbutton-vertical-text-padding: calc(var(--toolbarbutton-inner-padding) - 1px);

browser/themes/shared/browser-shared.css

@@ -30,6 +30,7 @@
   --toolbar-bgimage: var(--toolbar-non-lwt-bgimage);
   --toolbar-color: var(--toolbar-non-lwt-textcolor);
 
+  --tab-selected-bgimage: var(--tab-selected-non-lwt-bgimage);
   --tabs-navbar-shadow-size: 1px;
 
   --panel-shortcut-color: inherit;
@@ -114,6 +115,7 @@
   --toolbar-bgimage: none;
   --toolbar-color: var(--lwt-text-color, inherit);
 
+  --tab-selected-bgimage: none;
   color: var(--lwt-text-color);
 }
 

browser/themes/shared/tabs.css

@@ -556,7 +556,7 @@
 
 .tab-background:is([selected], [multiselected]) {
   background-color: var(--tab-selected-bgcolor, var(--toolbar-bgcolor));
-  background-image: var(--toolbar-bgimage);
+  background-image: var(--tab-selected-bgimage, var(--toolbar-bgimage));
   background-origin: border-box;
   background-repeat: repeat-x;
 }