webui: redable color of invalid fields on login-screen-like pages

Pages with widgets like LoginScreen, MigrateScreen use login-pf styling.
This page has dark background instead of light. Thus styling for labels
for fields with error has color which makes the label hard to read or
almost invisible.

Change it to white so it is still readable.

Fixes: https://pagure.io/freeipa/issue/7641
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
Reviewed-By: Petr Vobornik <pvoborni@redhat.com>
Reviewed-By: Serhii Tsymbaliuk <stsymbal@localhost.localdomain>

install/ui/less/login.less

@@ -29,6 +29,10 @@
     background: @login-bg-color url("@{img-path}/@{img-bg-login}") repeat-x 50% 0;
     background-size: auto;
     color: #fff;
+
+    .has-error .control-label {
+        color: #fff;
+    }
 }
 
 .reset-login-pf-height() {