Plano is a flat theme designed for GNOME and Xfce4 desktops.
Yeah, I know; it seems as though there are now more flat GTK themes for Linux than there are music players! That’s not to say Plano isn’t a nice theme — just look at it! — but it is ‘yet another’ iteration on a design trend du jour.
Part Arc GTK Theme, part modern Adwaita, Plano is gives a subtle look to apps and windows, with its muted colour palette ensuring it stays out of the way. I recommend using it with a dark icon theme (pictured above with Papirus) for a high contrast look.
Plano’s window controls do add a splash of personality and colour, but the bright red ‘close’ icon is unlikely to be to everyone’s tastes. Red as a colour generally conveys urgency, so the close button in this theme just makes me feel like I need to close apps ASAP!
The theme has a few missing bits, including one where switches and checkboxes are invisible in some situations/apps. It also requires GTK 3.22 or later, so if you’re reading this post from a reliable Ubuntu 16.04 LTS machine, you may wish to stick with Arc.
But if you like what you see you can download the latest release of the Plano GTK theme from Github:
When the download completes extract the folder and move its contents to ~/.themes.
Finally, to apply the GTK theme use GNOME Tweak Tool.
Tip: To use the included GNOME Shell theme you’ll need to have the User Themes GNOME extension installed and enabled.