Copy the .xml files to the styles folder of QT Creator.
Windows:
xcopy Material.xml C:\Qt\Tools\QtCreator\share\qtcreator\styles
MacOS:
cp Material.xml ~/.config/QtProject/qtcreator/styles/
Linux:
cp Material.xml ~/.config/QtProject/qtcreator/styles/
For better experience, change the theme color in Tools->Options->Environment to match the theme background color of the themes.