Update style.css
Update header.php
Update style.css
Update style.css
Update style.css
Update style.css

Using new CSS features.
Knowing issue: incorrect background separation if browser does not support flexible box layout and sidebar content is larger than blogposts.
Delete bodertop.png

replaced by css shadow
Delete navline-vert.png

replaced by CSS shadow
Delete content-bg.png

replaced by CSS
Update style.css
Merge pull request #6 from J0WI/master

Thanks a lot for your help!