A recent commit (00bce130d35e9dc398709e690a05f8dde70f52b3) modified the KMessageWidget look and feel. While I fail to see the point to make them look like their Kirigami equivalent this doesn't really bother me either.
The change did introduce 1 regression IMHO: the Information type background colour is now hardcoded instead of being based on the highlight colour as before. This is also the type I encounter most (in Kate, KDevelop etc).
The patch below reverts the colour hardcoding by obtaining the base colour from the current palette, as before.
Additionally, I propose to use the current widget palette instead of the global application palette, as we cannot know whether or not the code calling us installed a custom palette for the particular widget we're working with.
I've marked this WIP because I plan to use the occasion to investigate whether or not it's possible to drop some of the other hardcoded colours.
I am also open to discussion whether it would be more appropriate to use the user's tooltip colour (QPalette::ToolTipBase) for Information messages, or to introduce a type-specific darkening factor for the calculation of the final background colour.