With Qt you can use the Qt Style Sheets to specify global and item specific styles. These can be stored in a file (which may be also embedded into the Qt app) and loaded upon program start. The advantage is that all style information is located in a single place and you don't have to use the Qt Designer for each widget.
I did not found examples for flat applications with Qt style sheets. But it is always up to you to design your application and even with examples you have to know the basics to modify the examples and add your own parts.
Some links:
Qt Style Sheets Examples[
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Qt Style Sheets Reference[
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