The Eclipse IDE needs some extra hacks to improve its appearance. As there are
no CSS classes on the UI elements in question, the problem must be solved with
an external stylesheet. To apply the styles, add the following two directives
into your ‘eclipse.ini’:

-Dorg.eclipse.swt.internal.gtk.noThemingFixes
-Dorg.eclipse.swt.internal.gtk.cssFile=<PATH-TO-THE-STYLE>/eclipse.css

And then create the stylesheet with the following content:

toolbar {
  border: 0;
}
toolbar button {
  background: none;
}
toolbar button.flat:hover {
  background: #eeebe7;
}
toolbar button.flat:active,
toolbar button.flat:checked {
  background: #c4c2bd;
}
scrolledwindow {
  border: 0;
}
frame {
  border: 0;
}
