Datepicker: update styles after update to new version

[tahini]

With #1210, the react-datepicker package was update and the default react-datepicker css file was used instead of our modified file (as the style was not correct anymore)

Among other things, the colors can be updated to look as they used to. For information, the following commits from transition-legacy made changes to the _datepicker.scss file:

https://github.com/chairemobilite/transition-legacy/commit/8186a5d9bf
https://github.com/chairemobilite/transition-legacy/commit/bbb8c1f723bf2e27d6356027cc9a6dc35ae08818

Look before the update:

Look after the update: