Jetpack Scan: update style of greyed-out fields on iOS and desktop Safari

[v18]

After entering the SSH credentials and clicking on 'Save Settings' the greyed-out fields on iOS look different than those on other browsers.

Here's what it looks like on Desktop Chrome & Android Chrome (seems like this is what is intended):

Desktop Chrome:

Desktop Android:

The fields are greyed-out, but still readable. It looks the same on FF, and IE11.

On iOS (Safari and Chrome), the fields are greyed out and are hard to read. Here's what it looks like on iOS Safari:

iOS Chrome (same as iOS Safari):

Desktop Safari (a little bluer):

This also happens with other fields in Calypso, for example in the Settings > General page for the text fields 'Site Title' and 'Site Tagline' after clicking on 'Save Settings'. Here's an screenshot from iOS Safari:

Since the issue affects more than just the fields in Jetpack Scan, I added the Design and Framework labels, too.