Add css for dark mode

[joschne]

Current situation

global.scss defines dark mode variables only for --ion-color-tertiary.

Color tertiary is used in geov-entity in the background.

The download rdf button, positioned on such a background, is almost invisible in dark mode, as posted by @flicksolutions in #85 (comment)_

Reason: the button uses --ion-color-primary for which there is no dark mode variable set.

Goal

• Add dark mode variables for all variables, as in this example: https://ionicframework.com/docs/theming/dark-mode#ionic-dark-theme
• Integrate it in storybook to work correctly