QTcreator obeys your kde-wide configurations.
If you choose "obsidian-coast" as the system-wide color scheme qt creator will be all dark as well.
I know it is a partial solution but it works.
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]
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".
I found a way to change the Application Output theme and everything that can't be edited from .css.
If you use osX:
Navigate to your Qt install directory.
Right click Qt Creator application and select "Show Package Contents"
Copy the following file to your desktop> Contents/Resources/themes/dark.creatortheme
or /default.creatortheme. Depending on if you are using dark theme or default theme.
Edit the file in text editor.
Under [Palette], there is a line that says error=ffff0000.
Set a new color, save, and override the original file.