Some qt styles have a problem with displaying the block progress, as text on a segmented progress bar is terrible to read. This mostly affects older Windows, and some Linux distributions (which also use the windows/windowsxp theme).
Add a custom stylesheet as workaround, but only when one of those renderers is active, otherwise leave the theme alone (issue #1071).
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After:
@diapolo: can you test this on Windows 7 and see whether it stays as it is?