Add dark style to onboarding screen (fix #956)

Currently if we use Kresus, enable the dark mode and then go back to onboarding there will be issues like #956 (closed). Also, we should set the default value of the dark mode in regards to the browser/OS preferences and beginning at the onboarding already (I'll see to that in another MR but that will have an impact on the onboarding display in dark mode).

Below a screenshot of what my MR does. The left block shows how it appears on hover.