Yeah, that was the problem. I'm running KDE 4, and I'm using the Oxygen widget style.

Note below that I'm now setting the widget's style to QWindowsStyle, which is very basic, which seems to work.

Qt Code:
  1. QWindowsStyle * WindowsStyle = new QWindowsStyle;
  2. testLabel = new QLabel("Hello, world!");
  3. testLabel->setStyleSheet(
  4. "background-color: rgba(255, 255, 255, 0%);"
  5. "font-size: 20px;"
  6. );
  7. testLabel->setStyle(WindowsStyle);
  8. addWidget(testLabel);
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Thanks!

Bart