[j.1] highlight checked QToolButton using black background (#883) ()

Merging branch 'j.1'  into 'main'.

See merge request !2284

GUI/styles/Base.stylesheet

@@ -99,6 +99,11 @@ QToolButton#ViewSelectionButton:disabled
     border-right: 1px solid rgba(100, 100, 100, 25%);
 }
 
+QToolButton:checked
+{
+    background-color: black;
+}
+
 /* ------------------------------------------------------------------------------- */
 QRadioButton
 {