Bug 1494894 - sidebar CSS: use color instead of opacity for unselectable items;r=ladybenko

Depends on D9652

This allows the Connect button to no longer look disabled. However we could maybe mutualize
colors a bit more, either with a --color-disabled variable or with a class.

Differential Revision: https://phabricator.services.mozilla.com/D9653

--HG--
extra : moz-landing-system : lando

devtools/client/aboutdebugging-new/src/components/sidebar/SidebarItem.css

@@ -18,7 +18,7 @@
 }
 
 .sidebar-item:not(.sidebar-item--selectable) {
-  opacity: 0.5;
+  color: var(--grey-40);
 }
 
 .sidebar-item--selectable:hover {