QKdeTheme falls back to Qt::ColorScheme::Unknown on invalid desktop theme

QKdeTheme expects KDE settings to be stored in specific locations.
If an unsupported linux distribution stores theme settings elsewhere or not at all, reading the settings will return a default constructed QVariant. In this case, the color scheme will not be updated. When the theme name is found, but does not contain the key words "dark" or "light", the colorScheme is determined heuristically. This fallback doesn't happen, when no theme is found.

While harmless for supported platforms, this behavior could be changed to the benefit of unsupported ones.