* Update style.less
In chrome and safari the main-page content is not shown in the center of the page.
Adding a
display:flex
to the body-styles in the css-file fixes this problem.
* Update style.less
Building the site fails if the menu is not defined in the config,
because the `header.html` references `.Site.Menus.main`. It can be made
optional by using the `with` helper.