Fixes for themes v4

[csalmeida]

This update introduces a range of fixes and updates to themes for the v4 front-end of Protonmail.

• Fixes buttons with missing styles, such as primary and disabled.
• Fixes issue where full themes would not have have base colour applied to compose box on plain text emails (Light text doesn't display well when composing "plain text" emails #55).
• Styles message count next to mailboxes with theme colours.
• Minor style fixes for contacts.

[csalmeida]

Change "back to mailbox" button to have a theme coloured outline

[csalmeida]

Contact merge progress bar in modal has no theme colours

[csalmeida]

Use theme colours on contact merge highlight toast

[csalmeida]

Add theme styles to number of messages per box.

[csalmeida]

Light text doesn't display well when composing "plain text" emails