improve colors of login screen

improve colors of login screen

default primary color was a purple hue that did not really fit with our
theme. That was used on highlighting form fields and the progress
indicator. I replaced it with orange since that fits nicely with
the rest of the look and feel

also removed the use of accentColor int he ProxmoxProgressOverlay, so
that the text appears now white instead of some shade of cyan.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>