Can I override a single UI color, e.g. the selection background?

[SPITIRU]

The selection color is a bit too subtle on my monitor. Can I change just that color without forking the theme?

[ziqq]

Yes, use workbench.colorCustomizations scoped to the theme so it does not affect other themes:

"workbench.colorCustomizations": {
  "[CodePen Theme Original]": {
    "editor.selectionBackground": "#4a5568aa"
  }
}

Any key from the Theme Color reference works the same way.