Adjust tree node foreground color when active

Fixes #3960
Fixes #6831

With this change, when a tree node is active, both the content including the node itself,
the `tail` and `suffixes` are displayed with the same `activeSelectionForeground` which helps in
visibility, the overall look, and also aligns with VS Code. Previously, for
certain themes, the colors were difficult to see.

Signed-off-by: vince-fugnitto <vincent.fugnitto@ericsson.com>

packages/core/src/browser/style/tree.css

@@ -85,7 +85,14 @@
 .theia-Tree:focus .theia-TreeNode.theia-mod-selected,
 .theia-Tree .ReactVirtualized__List:focus .theia-TreeNode.theia-mod-selected {
     background: var(--theia-list-activeSelectionBackground);
-    color: var(--theia-list-activeSelectionForeground);
+    color: var(--theia-list-activeSelectionForeground) !important;
+}
+
+.theia-Tree:focus .theia-TreeNode.theia-mod-selected .theia-TreeNodeTail,
+.theia-Tree .ReactVirtualized__List:focus .theia-TreeNode.theia-mod-selected .theia-TreeNodeTail,
+.theia-Tree:focus .theia-TreeNode.theia-mod-selected .theia-caption-suffix,
+.theia-Tree .ReactVirtualized__List:focus .theia-TreeNode.theia-mod-selected .theia-caption-suffix {
+    color: var(--theia-list-activeSelectionForeground) !important;
 }
 
 .theia-Tree .theia-TreeNode.theia-mod-selected {