gimp/themes/Compact/compact.css

/* Set of common styles for the Compact theme for GIMP 3.0 */

* {
  -GimpToolPalette-tool-icon-size: small-toolbar;
  font-size: small;
}

toolpalette button {
  padding: 0px;
}

button, tab {
  padding: 0px;
}

paned separator {
  padding: 1px;
}