Fix styling for remaining widgets in Gtk 3.20

[xyl0n]

The port for Iris Dark is roughly 40% done, this is just a list to help me keep track of what's left and needs further styling (note that Light and Night still need a complete redoing):

• InfoBars
• Pathbars
• Dialogs
• Popovers
• Spinbuttons
• Some linked buttons (e.g. in headerbars)
• Treeviews
• Sidebar rows
• Overshoots
• OSD
• Active Entries
• Notebook
• Insensitive buttons
• All assets
• Scale and Progress
• Flat buttons (all states, e.g. checked)
• Selected Buttons
• Selection mode

(When I put it like this, there's actually quite a lot of stuff still left isn't there? :/ )

[Vistaus]

Any updates on this? I miss Iris (and Iris Light) :(