Comment 1 for bug 1498581

To get an icon which fits all themes very well is a very difficult or rather impossible task. In the past we used a default icon of a theme but in many cases that also looked out of place.

So what we do is with Diodon we define a list of themes where the monochrome icons fit. If the theme is not in that list a Diodon icon is used.

You can have a look here https://esite.ch/2014/07/where-is-my-diodon-monochrome-icon/ for more details and how you can change the icon on your machine as well.

In case one of the icons already shipped with Diodon fits well with your used theme, feel free to open up a bug report so we can add that theme to our list.

In case the theme you use is not so common and the icons do not fit it might be better to ask the theme author to ship a clipboard (Diodon) which fits their theme.

Hope this helps.