Refactor auth form styles

Closes #4246

- Removed the input wrapper background colour
- Moved `:focus` styles to one element, so its not defined separate for each page
- Removed unnecessary MQ for body background colour (it changed nothing)
Merge pull request #4249 from PaulAdamDavis/fix-signin-styles

Refactor auth form styles
Merge branch 'master' of https://github.com/RaoHai/Ghost into keyboard-shortcut-context