exp_adminui 1.0.0.42: a readable sign-in button

• The sign-in button of the Admin UI login page had a white label on light coral (2.6:1). It takes the Admin UI's filled accent (--a4-accent-strong, hovered --a4-accent-press), white on it above 5.5:1, with a focus ring on the dark frame.
• Nothing has to change in an installation.