Storybook doesn't apply all CSS styles

[lindapaiste]

1. The modal needs to have a parent with class="light" (or "dark" or "contrast"). This will be fixed by Add theme switching to Storybook #2326, but we can also put in a quick fix until that gets merged since it might be a while. We can add a <div className="light"> to the decorators in preview.js.
2. The compiled main.css file that we are importing in preview.js is not up to date. I don't know what that file is for and probably it should be deleted. We want to import the uncompiled file import '../client/styles/main.scss' as that will be current will all changes. But in order to do that we also need a Storybook addon to process it: https://storybook.js.org/addons/storybook-addon-sass-postcss

Originally posted by @lindapaiste in #2377 (comment)

[lindapaiste]

fixed by @ofhope in #2388