set insensitive bg color to red to spot problems
insensitive state fix
  - use proper $color for insensitive button background
more insensitive state fixes
  - set the correct bg to backdrop insensitive buttons
  - remove red and use a brighter then bg color gray for insensitive bg
added .header-bar-separator to the relative section
added and used around $selected_borders_color;
headerbar: explicit separator class as in master
osd button: FIXME - lapo why? ;)
assets: no more popover assets
initial dialog styling
osd: initial osd styles
radio, check: render new assets
calendar: basics
headerbar: separator needs two selectors :(
notebook: lighten up the header

- lighter bg color
- no text-shadow. just makes the label fuzzy.
notebook: reorderable active tabs should still be blue
scale: fix dark insensitive backdrop color
popover buttons: don't lighten up, use $bg_color
inline-toolbar: sync bg color with notebook header bg
headerbars: lighter and flatter
sidebar: basics