CURA-4380 quality slider text disabled color when slider cannot be used

resources/qml/SidebarSimple.qml

@@ -309,7 +309,7 @@ Item
 
                     text: catalog.i18nc("@label", "Slower")
                     font: UM.Theme.getFont("default")
-                    color: UM.Theme.getColor("text")
+                    color: (qualityModel.availableTotalTicks > 0) ? UM.Theme.getColor("quality_slider_available") : UM.Theme.getColor("quality_slider_unavailable")
                     horizontalAlignment: Text.AlignLeft
                 }
 
@@ -320,7 +320,7 @@ Item
 
                     text: catalog.i18nc("@label", "Faster")
                     font: UM.Theme.getFont("default")
-                    color: UM.Theme.getColor("text")
+                    color: (qualityModel.availableTotalTicks > 0) ? UM.Theme.getColor("quality_slider_available") : UM.Theme.getColor("quality_slider_unavailable")
                     horizontalAlignment: Text.AlignRight
                 }
             }