Themes: explicitely set background and text color for tooltips

As the colors of QToolTip depend on the desktop environment used,
explicitely set background and text color for fixing cases like
white text on bright yellow background.

Closes #3006955.
(cherry picked from commit d7ce7d485b)

data/themes/default/style.css

@@ -12,6 +12,12 @@ automationEditor {
 	background-color: rgb(0, 0, 0);
 }
 
+
+QToolTip {
+	background: rgb(224,224,224);
+	color: black;
+}
+
 QMenu {
 	border:1px solid black;
 	background-color: rgb( 192, 192, 192 );