MUCJoinLeft: Use brighter colors in dark mode

gajim/data/style/default-dark.css

@@ -28,10 +28,10 @@ .gajim-state-tab-muc-msg {
     color: rgb(80, 95, 230);
 }
 .gajim-user-disconnected {
-    color: rgb(80, 30, 30);
+    color: rgb(230, 170, 170);
 }
 .gajim-user-connected {
-    color: rgb(30, 80, 50);
+    color: rgb(160, 200, 170);
 }
 .gajim-roster-disconnected {
     background: rgb(80, 30, 30);