Customize emacs leuven-theme – remove over- and underlines

Some people, including myself, don’t like the over- and underlining of headlines or sourceblocks defined by leuven-theme. Over- and underlining can be removed easyly. You only have to overwrite the matching attributes after loading the theme, this can be achieved with the set-face-attribute function. I use use-package to load emacs packages, here is the complete