Well, to round the windows I used lightlyshaders, and for the window decorations (which includes the outline) I’m using a modified version of Klassy to make it compatible with the rounded windows. Unfortunately, the latest versions of these two are no longer compatible with each other (though they still work individually). My theme started out as the Dracula theme from the KDE store, but I modified it a bunch to add some stuff from other themes. The version shown in the screenshot is mostly the “Utterly Round” theme with the colors changed to the ones from Dracula. The window open/close animation is the burn-my-windows TV effect.

It’s the burn-my-windows TV effect.