Fix contrast in UI for readability/accessibility

[Hukuma1]

I like the color scheme of Umbrel, but the colors do not pass contrast checks.

The green login button for instance. If those colors are run by any contrast checker, it would fail:

https://buttonbuddy.dev/#generator

--surface: #f7f9fb;
--background: #00cd98;
--color: #fff;
--focus-background: #00a77c;
--focus-color: #fff;

The disabled color of the green button is even more evident of this issue. The first time I was trying to pass the start screen I wasn't aware there even was a minimum password length requirement. The button's visual change is too mild.