qtdeclarative example needs to be updated for dark themes

Examples have mostly been written in the time when there was no such thing as a dark-theme. Today a large percentage of developers actually runs with a dark theme.
Should you have a KDE or similar setup which stores the colors in the user-global Qt config, this affects the examples.

The PR on https://codereview.qt-project.org/c/qt/qtdeclarative/+/438464
fixes an example to no longer have hardcoded colors which resulted in white text on a white background for those using a dark-theme.