A first step to adding automatic dark mode color scheme switching. Adds a new SCSS file at `color_definitions.scss` that serves to output all SCSS color variables as CSS custom properties. And replaces all SCSS color variables with the new CSS custom properties throughout the stylesheets.
This is an alpha feature at this point, can only be enabled via console using the `default_dark_mode_color_scheme_id` site setting.
* DEV: Remove the margin from widget-dropdown
Generic components should not have a margin. Those should be styled in the place where they are used.
* DEV: Remove margin from the dropdown body
It triggered a warning in popper and was effectively a no-op as popper positions dropdowns on its own using `position: fixed` and `top/right/bottom/left` properties.
