UI: Fix disabled items in Dark theme being too light

In the Dark theme, disabled UI elements had text that was too close in
color to enabled UI elements. This commit switches them to use the
"light" palette color instead of the "lighter" palette color.

UI/data/themes/Dark.qss

@@ -55,9 +55,9 @@ OBSTheme {
 }
 
 OBSTheme::disabled {
-    text: rgb(200,199,200); /* lighter */
-    buttonText: rgb(200,199,200); /* lighter */
-    brightText: rgb(200,199,200); /* lighter */
+    text: rgb(122,121,122); /* light */
+    buttonText: rgb(122,121,122); /* light */
+    brightText: rgb(122,121,122); /* light */
 }
 
 OBSTheme::inactive {