Improve color contrast

During the accessibility audit #341 several issues were identified concerning poor color contrast:

• registration screen: text field label (light blue on white/black background) > !213 (merged)
• settings/change password screen: text field labels (light blue on white) > !213 (merged)
• settings screen: "change password" button (light blue on white/black) > !213 (merged)
• registration screen: "next" button (black text color on blue/gray (disabled) button) > already fixed on current main
• registration screen: visual focus indicator on "show/hide password" button (light gray on white)
• registration screen: password strength indicator (light green on white / dark red on black) > !211 (merged)
• registration screen: form error color (red on black) > !202 (merged)
• message compose screen: send button (light green on white) > !212 (merged)

these two issues were post-poned to #364:

• registration screen: disabled "next" button if some of the fields are not properly filled (light gray on white/black)
• settings/change password screen: disabled button (light gray on white)

as well as:

• add contact screen: close button (system-dependant window decoration, so I don't think we can/should do anything about it)