See finding  in the Prüfbericht Barrierefreiheit GnuPG Kleopatra 188.8.131.52
The standard Breeze icons (used by Kleopatra) are very dark because they are supposed to be used with a light color scheme. This can be "fixed" by the user by switching from the Breeze icon set to the Breeze Dark icon set.
But for this we would have to include the Breeze Dark icon theme in the installers and we should probably add a setting for switching between light and dark mode (at least on Windows; on Linux the users will anyway set their preferred theme for the whole desktop).
@aheinecke What do you think?