[5.x] Add dark mode to saving overlay on ChangePassword component

[martyf]

The saving overlay that is used on the change password popover does not support dark mode:

This PR resolves this by doing two things:

1. removes the white colour from the .saving-overlay class, and
2. replaces it with Tailwind colours in the ChangePassword.vue to make it easier to work with the dark mode selector.

I believe this is the only piece of code that uses the .saving-overlay class.

Result:

This is using dark:bg-dark-600, but dark:bg-dark-800 isn't too bad either.

[duncanmcclean]

Oh, nice! 💅

[jasonvarga]

Thanks! I tweaked it to just keep it in css. I know we only use the class in one place at the moment, but if we wanted to use it somewhere else we wouldn't have to apply extra styling classes to it.