The concept of ‘coloured’ decoration themes seems to me to be a bit too much of a dinosaur - unless you really want it fixed and not responsive to your colour scheme…
Also, with Klassy you can modify the appearance of the buttons.
They have Plastik as one of the presets.
You can set the shape to ‘rounded square’.
You can also save your settings as presets, so I can easily switch from this one…
and back again.
I can also over-ride the button colours - as you see, I am in my ‘colourful’ phase, and everything blends with the colour-scheme (hence the green tint to the ‘active’ window text).