core styling to match figma. The inside bar is not positioned yet, that will be addressed in the next diff.
https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/Comm-%2F-Desktop-app?node-id=1170%3A77791
https://linear.app/comm/issue/ENG-530/inline-sidebar-re-style
{F49373}
{F49374}