Bring back light theme

Hi there,

#707 makes the light theme unavailable. However, some people might like it so we might want to bring it back.

> We are using the dark theme as a default now, because all the (former) enterprise plugins only implemented that one.
>
> Because the dynamic plugin loading subsystem only supports regular CSS stylesheets, there is no way to align this with SCSS stylesheets used for the general site. However, to see the UI using the light theme, SCSS variable modifiers are used. So, it will need another iteration to refactor those plugins into regular views to make their stylesheets be able to use SCSS.

The switch is in `app/styles/styles.scss`.
```scss
// TODO: Add a switch for selecting light vs. dark theme.
// @import 'vars/light-theme-variables';
@import 'vars/dark-theme-variables';
```

With kind regards,
Andreas.