I found some trick for your problem! Here you can see it: Habrahabr -- Redesigning Qt Creator by your hands (russian lang.)
According to that article, that trick is kind of not so dirty, but "hack" (probably it wouldn't harm your system, but it can leave some artifacts on your interface).
You don't need to patch something (there is possibility, but I don't recommend).
Main idea is to use stylesheet like this stylesheet.css:
// on Linux
qtcreator -stylesheet='.qt-stylesheet.css'
// on Windows
[pathToQt]\QtCreator\bin\qtcreator.exe -stylesheet [pathToStyleSheet]
To get such effect:
To customize by your needs, you may need to read documentation: Qt Style Sheets Reference, Qt Style Sheets Examples and so on.
This wiki page is dedicated to custom Qt Creator styling.
P.S. If you'll got better stylesheet, share it, I'll be happy! :)
UPD (10.12.2014): Hopefully, now we can close this topic. Thanks, Simon G., Things have changed once again. Users may use custom themes since QtCreator 3.3. So hacky stylesheets are no longer needed.
Everyone can take a look at todays update: Qt 5.4 released. There you can find information that Qt 5.4, also comes with a brand new version of Qt Creator 3.3. Just take a look at official video at Youtube.
So, to apply dark theme you need go to "Tools" -> "Options" -> "Environment" -> "General" tab, and there you need to change "Theme".
See more information about its configuring here: Configuring Qt Creator.