Update to the headerbar and error color

Make the headerbar follow the usual gnome app design instead of being flat. This is for consistency across the desktop, it's also generally better for accessibility reasons with the color contrast and separator in light mode. See 9fc49198 for a preview.

Change the warnings' color to @error_color, this undoes the change requested in #77 (closed). We want to avoid hard coded colors as much as possible, and follow adwaita for consistency with gnome. It's better to have the same color palette across the DE so that users with problems can easily change them around with tools like gradience. This color also adjusts automatically for the dark mode which is currently too abrasive.